You may not be guilty of murder for fraudulently acting as a man entombed under 400kg of concrete, but it sure raises some questions.

That's the hidden detail that until now the New Zealand public have been unaware of in the case of David Hart - whose remains were found under his Mt Eden home on January 31 last year.

The bones had laid under a mound of concrete on the dirt floor of a basement workroom in the Marlborough St property since early 2004.

The door had been Gibbed securely shut, and for the next 12 years a ramshackle boarding house in the weatherboard double-storey building above continued to operate.

Ex-crims and elderly alcoholics were the typical clientele of the cash-only lodgings, rarely settling for long.

The only difference is that Hart was no longer the landlord.

In his place was an existing tenant who had spent time in a maximum-security prison in the 80s for violent crimes, who continued to pay the rates on the property until he died in 2016.

The detective overseeing the case, Senior Sergeant Glenn Baldwin, this week spoke to the Herald about the difficulties of a cold case the likes of which he has only seen once before in his career.

"It's one of those fascinating, really challenging cases: what happened to David Hart?" Baldwin said.

"It is sad that nobody missed him for all of that time for whatever reason. He was an interesting, eccentric character.

"Our [crime] scene was very limited and that's often a start point for a criminal investigation let alone an investigation into the death of somebody.

"The house was completely gutted. We did luminol [chemical blood search] the scene in the advent that might yield something but the scene has undergone a massive renovation.

"We're always playing a little bit of catch-up. The time for a body to become skeletal… it was obviously not something that had happened overnight but some time ago."

Nevertheless, Baldwin this week revealed a key piece of evidence that allowed police to elevate the case to a homicide investigation and identify a "person of interest" in Hart's murder.

It relates to a second property that Hart owned in Blackball on the West Coast, where he used to travel to with his brother in the 70s and 80s to gold prospect.

Hart's nickname on the Auckland Waterfront where he worked as a tally clerk was "the Gold Digger".

Baldwin said it was mentioned among several of the old Marlborough St residents police spoke to that he may have had gold nuggets in his possession, which he on rare occasions showed other boarders.

"There was talk he might not have been cashing up the gold. An urban myth was that he collected some of the gold. He was not super trusting of banks - but, that said, he used banks."

Hart's last transaction for his ASB account was March 2004 and after this point Baldwin says police have found no one with any recollection of seeing Hart.

However, in early 2005 someone purporting to be Hart tried to sell his Blackball property.

This person had contacted the Grey River council and tried to have the rates of the Blackball property transferred into the potential West Coast buyer's name.

"When our investigators were down there they made contact with this person who had tried to acquire the property and they still had some correspondence from back then, which was fortunate for us," Baldwin said.

"The document itself was typed but the envelope was handwritten so we were able to get that handwriting analysed and we were able to get some sample handwriting of our person of interest and verify that that person of interest was the author of those letters purporting to be Mr Hart."

That person of interest is the same Marlborough St tenant who took over control of the boarding house after Hart's disappearance in 2004.

This person spent time in the 80s in Paremoremo maximum security prison in Auckland for violent crimes.

"He had a violent past," Baldwin confirmed.

The man died in 2016 of natural causes at which point the Marlborough St property went into rates arrears and was sold in 2017.

"If he was alive now he would be someone we would definitely like to speak to. I think he would hold a number of answers for us," Baldwin said.

"Our person of interest may well be and probably is guilty of fraud offences. But as far as being culpable for death, there's an evidential gap there for us. He's certainly someone who would appear to know David Hart's not around or probably dead. But again that's still short of being able to prove to a high standard he's responsible for the death.

"It doesn't exclude him, but we don't have the evidence to be able to say that.

"I think it is important the public understand there's not a random killer running around Auckland. Our person of interest had a link to the address, as did our deceased."

Baldwin says police are particularly interested in the period from late 2003 to early 2004 and are still seeking tenants at the Marlborough St boarding house from that period.

"I would be surprised if there weren't some behaviours at the time of David's death that didn't elevate somebody's suspicions or concerns about something being not quite right," Baldwin said.

The police investigation has so far spoken to "a lot" of past residents as far away as the UK and France.

Two figures of particular interest are a man described as the "Polish guy" and an Asian woman who police believe was a friend of Hart.

"It's not clear whether she may have been a partner of David or not, or whether or not she might have just been somebody that visited the address, or a short-term resident," Baldwin said.

"At this point we don't have a name, we don't have anything other than people referencing a shorter woman, sort of a plump build, Asian, possibly Thai or Indonesian. We'd like to identify and speak with her."

The delivery of 400kg of concrete to the Marlborough St house is also one of the main leads police are keen to get more information about.

"I think this concrete has been poured over David's body to entomb him. That would take some time," Baldwin said.

"If the concrete is mixed in a wheelbarrow, there's some effort in mixing 400 kilos of concrete, so if that triggers some memory or something about some unusual behaviours or things that were going on at the address we'd really like to hear from those people."

Police would like to hear from anybody who knows anything about building supplies delivered to 3 Marlborough St around 2003 to 2004.

Other details police have uncovered of Hart's life are sketchy, but Baldwin can reveal he had undiagnosed diabetes, which may have caused him mood fluctuations.

"I guess interpreting and trying to understand our victimology - [he was] potentially difficult to deal with, could be abrasive, but then could be quite enthusiastic to see and talk to people, perhaps to the point of 'I'm ready for this conversation to be over now'. That's how some of the neighbours portrayed it. So I think at times could be friendly.

The identification of Hart's remains on September 4, 2020 came courtesy of a blood sample of his brother, retrieved from a district health board when he died five or six years ago from cancer.

DNA evidence from the Mt Eden bones connected them to this DHB blood sample. Both of them shared the same father.

Hart would have been 80 today if he were still alive.

After getting in contact with some of Hart's living nieces and nephews, it was uncovered he was estranged from his brother for decades after a bust-up at their father's funeral.

"Sadly nobody missed poor old David. Isolated from his family, I don't think there was anything specifically malicious about that."

Baldwin says a year of Covid-19 strained police resources, but the investigative team overseeing the Hart case met early this month to discuss the next steps.

He believes people out there still have information and urges them to ring (09) 302 6557.

"David Hart didn't deserve to die the way he has obviously died. I don't know the cause of death but I know he's ended up buried under concrete and nobody deserves that," Baldwin says.

"I believe he has been killed and somebody is responsible for that. He might have been at times difficult to deal with but he didn't deserve that. People who may have known some of the characters living at the house may have some apprehension and fear of those people.

"Our person of interest is deceased so if somebody has information they can come forward safely. We really encourage people to do that.

"Do the right thing for David. He deserves to have his story told and understood."