Reminder, this is a Premium article and requires a subscription to read.

Kayo Matsuzawa dreamed of travelling to New Zealand. Photo / TVNZ

Kayo Matsuzawa was on her dream trip to New Zealand where she planned to study, work and perfect her English. But the 29-year-old was murdered just two months before she was due to return home. Twenty-four years later, police have still got their sights set on finding whoever is responsible.

He was there to test the fire alarms.

He'd been there before, made his way through the rabbit warren-like Centrecourt building, up the staircase to the utility cupboard off to the side.

Getting a key from the building manager was a pain, so he'd worked out a way to jemmy the door latch open with his screwdriver.

Pop the door, check the alarm, move on to the next job.

That's how it usually went.

When Dennis Groves opened the cupboard door he knew instantly something was not right.

What was that smell? Ammonia?

It was overwhelming, cloying, and Groves would later recall, "really vile".

He looked down and saw what he initially thought was a mannequin lying on the floor.

To his horror he soon realised he was looking at the naked and badly decomposing body of a young woman.

Groves backed slowly out of the stinking space and pulled the door shut.

He made his way back out to Queen St, undoubtedly sucking in as much fresh air as his lungs could take once he was in the open.

He pulled out his cellphone and called 111.

Neither Groves or police knew at the time, but that call marked the beginning of one of New Zealand's most bizarre unsolved crimes.

Kayo Matsuzawa was born and raised in a small fishing village in the Yamagata prefecture on Honshu, Japan's main island.

From an early age Kayo's biggest dream was to travel - and she always spoke of wanting to visit New Zealand.

In November 1997 she set off for a year in God's own - flying to Christchurch with a plan to explore the South Island, to perfect her English, to work, to have a real Kiwi experience.

Kayo's family were not keen on the idea of her travelling so far away, for a whole year on her own, but she convinced them she would be okay.

And she was, to start with.

Kayo lived her dream, studying hard, working hard and making friends.

In August 1998 she sent a postcard to her mother for her birthday.

"Happy birthday! I've decided to come home to Japan on November 4, so another three months to go … Take care. Love, Kayo.'

Before she left New Zealand she wanted to spend time in Auckland and the Bay of Islands.

Kayo invited her flatmate to join her in her journey north but the woman could not afford it.

She told Kayo that she would be safe, that many tourists travelled solo and that in New Zealand there was nothing to fear.

At about midday, Ansett flight AN626 touched down in Auckland from Christchurch with Kayo onboard.

The 29-year-old grabbed her bags and made her way to a shuttle bus, paying for a ticket into Auckland's CBD.

Security cameras tell the next part of Kayo's story.

2.41pm: Kayo gets off the shuttle and crosses Queen St.

She was booked into the Queen Street Backpackers hostel on Fort St and she checked in, placed her bags tidily on her bed and then headed out.

3.32pm: Kayo is captured walking past the BNZ building across the road from the backpackers.

Those images, grainy and fleeting, would be the last ever captured of Kayo.

She never passed those cameras again, she never returned to her room.

Something horrendous happened to Kayo that day, someone killed her, stripped her slight body naked and dumped her in a cupboard barely anyone knew existed.

Kayo lay there for 11 days until Dennis Groves chanced upon the terrible scene and alerted police.

To this day, what happened to Kayo after she left her room is a mystery.

But someone out there knows something - and even 24 years on police have still got their sights set on finding whoever is responsible.

There are just short of 70 unsolved homicides in New Zealand, dating back to 1914.

Most of the time arrests are made swiftly and offenders prosecuted, but there are some enduring and mysterious cases that are almost completely cold.

Kayo Matsuzawa is one of those. Despite exhaustive police investigations, reviews, media coverage and pleas from authorities and her family - we have little idea of how Kayo died, and who is responsible.

In a city like Auckland, in a country like New Zealand - how can a woman just disappear off a busy street like that?

With not a single witness?

Those are questions that Inspector Mark Fergus asks himself. Often.

Back in 1998, Fergus was a detective based in Auckland central and he oversaw the initial investigation into Kayo's death, dubbed Operation Net.

"I was involved from the very beginning," he said when he last spoke to the Herald about the case.

"I was asked to lead the general inquiries team - you have a number of teams in a homicide - the suspect team, area canvas, and in my case general inquiries which are inquiries that can relate to any of the circumstances at all - who was in the building at the time, who was around, who was using the card access systems [in the building where Kayo was found]."

"There were multiple parallel inquiries going the whole time but at the end of the day nothing quite gelled enough to really point us conclusively at any one suspect."

Fergus said the investigation was challenging from the outset.

Firstly, because while the police had a victim they had no idea who she was.

Nothing was found with the body - no clothes, no handbag, no jewellery.

There were no missing persons reports that matched.

Police released some information to the media - that the victim was a woman, young and Asian.

They got a call from the council that led them to identify Kayo.

Five days after she arrived in Auckland a rubbish collector had emptied a bin on the corner of Albert and Swanson Streets - behind Centrecourt and BNZ - and spilled some of the contents on to the concrete.

He picked up a handbag, purse, passport and insurance papers and handed them into the office before he finished work for the day.

Once reports of the unidentified body hit the headlines an office administrator contacted police and alerted them to the items.

Police then tracked Kayo to the backpackers - about 50m away from Centrecourt - and, using fingerprints from the luggage in her room and dental records, identified their mystery victim.

Her older brother Junichi was then flown to New Zealand to identify Kayo in person.

Another aspect of the homicide that hampered police was the length of time her body had remained hidden away in that locked room.

It was clear Kayo was the victim of foul play but after every conceivable stone was unturned there was no one who heard or saw anything, no one they could connect to both the victim and where she was found.

Police don't even know for sure how Kayo died.

"One of the challenges with this inquiry is because of the state of decomposition we've never really been able to nail down the cause of death," said Fergus.

"There were no obvious injuries, but that still doesn't discount causes of death that don't leave obvious marks, for example was she strangled? Was she subject to drugs? Was she asphyxiated?"

There have been countless theories about what happened to Kayo over the years. Did Kayo have her drink spiked at either a foodcourt or bar adjacent to the Centrecourt building.

Did she head out for a late lunch and unwittingly consume something that would have rendered her incapable of fighting off an attacker?

Did she meet someone at the hostel, in the street, at the foodcourt or bar and did they lead her to that utility cupboard for nefarious reasons?

It's almost impossible she would have mistakenly wandered there, as the area was out of the way and needed special key card access.

Was she killed elsewhere and dumped in the cupboard later?

These are all questions police are desperate to answer. Still.

Alongside theories, there have been suspects along the way.

There was a Ukrainian man who'd been a long-term resident at the Queen Street Backpackers Kayo had checked into.

He was eccentric and paranoid and had a history of mental health issues.

After police were told an older "dishevelled" man matching his description had been seen walking with an Asian woman the day Kayo arrived in Auckland, they honed straight in on the Ukrainian.

Suspiciously, they found that he had left New Zealand the very day Kayo's body was found.

LISTEN TO HERALD TRUE CRIME PODCAST A MOMENT IN CRIME HERE:

Police would later reveal that the man, who was a second-hand jewellery dealer, had pawned a number of items in Australia soon after.

The items matched jewellery belonging to Kayo that was not with her body, at the backpackers or with her dumped belongings.

An officer was sent to Australia to investigate further, but it was determined the items were not Kayo's.

Police issued an alert with Interpol - a global inter-governmental organisation that enables the sharing and access of data on crimes and criminals to member countries.

Two years later that alert was triggered when the Ukrainian was found by French police squatting in an airport.

He was extradited back to New Zealand, where Operation Net police spent two days questioning him about Kayo and his movements in September 1998.

The man was later released.

Police were confident that despite their earlier instinct, he was not the killer.

In 2000 the Herald reported on another potential suspect - Royal Navy Petty Officer Alan Michael Grimson, who at the time was facing two murder charges in the UK.

Police there have always suspected Grimson had more victims and was a serial killer.

What connected him to Kayo was the fact that Grimson was in New Zealand when she was killed - based at the Devonport Naval Base fire school as a trainer.

When news broke that Grimson had been charged - he was later convicted and sentenced to life in prison - police here confirmed he was on their radar too.

They had always said that Kayo's killer had access to Centrecourt and knew about the alarm cupboard.

We then learned that the manager of Wormald - the company that maintained the alarms and employed the man who found Kayo's body - knew Grimson.

Graham Osborne confirmed to the Herald at the time that he had met Grimson a few times at the fire school, and found him to be a "normal sort of guy".

But he believed his brief encounters with Grimson were coincidental and there was no reason for him to have known about the Centrecourt building.

Police requested that Interpol obtain information on Grimson's movements from British police.

They said that while he had not been ruled out, they were not getting excited about Grimson because he targeted young men and they had no evidence linking him to Kayo.

But like all other tips and leads - the information on Grimson was not ignored.

In 2018 police in Auckland got a tip from investigators in another district about a man who they thought could be of interest.

At the time the investigation team wouldn't elaborate on the tip further, but did confirm the man "looks really good" as a suspect.

They revealed that bank records showed the man had used a bank card at Queen Street's BNZ building ATM on the same afternoon that Kayo arrived.

Detective Inspector Scott Beard, the current officer in charge of Operation Net, said since the tip came in police had done extensive inquiry work around the man and his movements. But nothing they did led to an arrest.

"The investigative work we did in regard to this person of interest neither advanced or eliminated that person, so he is still a person of interest along with a number of others," said Beard.

Beard is ever hopeful his team can crack the case, but says they need help.

"The investigation is still open and occasionally we receive bits of information that we try to follow up," he said.

"As recently as this month we have made some inquiries.

"To solve this case, we would need either a key witness who knows or has spoken to the offender, or for the offender to decide to unload the burden they are living with. "That could happen due to a number of reasons including life or health change." Beard said Kayo's family remained desperate for answers. Her older brother Junichi, who was flown to New Zealand to identify Kayo's body in person, spoke briefly on TVNZ crime documentary in 2018.

"There is no time that I don't think of her. I think of her every day," he said.

"She was my sister, but she was also my friend."

Beard makes sure Kayo's family are updated when there is any new information.

Part of what drives him as an investigator is to get answers for families.

Arrests, charges, justice - that's part of it too, but at the very heart, it's piecing together Kayo's final moments so her family are no longer tortured by the unknown.

Along that vein, he had a simple message for the killer, or those close to the killer who may know what happened to Kayo all those years ago.

"Something like this will be playing on the individual's mind," he said.

"You might be at a stage in life where you need to speak to someone or clear your conscience.

"No one knows your circumstances at the time so police are not judging, we just want to resolve the case, especially for the family." Inspector Fergus, now working in a different role in the police, also implored the person who ended the life of Kayo Matsuzawa to end the mystery.

"Someone out there knows who killed Kayo," he said.

"The case of Kayo is still being investigated, leads are still being followed up and it often takes just that one little call to us, one whisper...

"It's pretty simple, it's time to come in."

There is at least one person out there who knows what happened to Kayo.

There could be more people who saw, found or noticed something in September 1998 that they never told police about.

If you can help solve the mystery of Kayo Matsuzawa, to help bring her family closure and her killer to justice - please come forward.

To pass on information contact the Auckland City Police on 09 3026400 or if you want to speak anonymously you can call Crimestoppers on 0800 555 111.

Reminder, this is a Premium article and requires a subscription to read.

NZTA Waka Kotahi warns motorists to expect delays.