Beware ambitious police officers with a sense of infallibility, warns veteran barrister Barry Hart.

Mr Hart says one of the most prominent police of the Crewe era was involved in faking a confession by one of the barrister's clients.

His comments come after Rochelle Crewe - a toddler when her parents were murdered in 1970 - and former Crewe inquiry detective Ross Meurant called for the infamous homicide case to be reinvestigated.

Police union chief Greg O'Connor also wants the inquiry relaunched, saying the case has been a black mark against the force.

Mr Meurant described in last week's Weekend Herald a "ruthless" culture under which he came to believe that because the police cause was just, the end justified the means.

In the Crewe case, a royal commission concluded that inquiry head Bruce Hutton and detective Len Johnston planted a shell case to frame Arthur Allan Thomas in what its chairman described as "an unspeakable outrage".

The Solicitor-General at the time, Paul Neazor, concluded there was insufficient evidence to bring charges.

Mr Meurant named Mr Hutton and another detective on the Crewe case, John Hughes, as leaders of a prevailing "macho" culture.

Mr Hart was involved in two subsequent cases in which Mr Hughes' conduct was called into question.

One so astounded Mr Hart that the lawyer has kept the case files. His client was Leo "Beaver" Ngamu, a robbery suspect. In an attempt to get a conviction, a false confession was produced and a police officer committed perjury when questioned in court about it.

"He [Hughes] wrote it all out," Mr Hart said yesterday. "It was supposed to be the guy's confession. It was all bullshit."

A young detective was taking a statement from Ngamu when Mr Hughes arrived, saw it didn't contain a confession, screwed it up and wrote a new statement with Ngamu's "confession".

Unseen, Ngamu picked up the discarded statement, which Mr Hart was able to produce in court to contradict the young detective's evidence that there had only ever been the "confession" statement.

"In my career, that was one of the worst situations ... dreadful," Mr Hart said.

The judge recommended the apparent perjury be referred to the Commissioner of Police, but Mr Hart says the commissioner effectively rubber-stamped his officers' behaviour. "The commissioner said he had looked into it and he had counselled both of the policemen. That's all that ever happened. They should have been charged with perjury."

Mr Hart says his client was pleased the case against him had collapsed and didn't want to create waves by pressing the matter further, so the issue of the fake statement died.

The barrister says police malpractice still occurs.

"What they do is work up a theory... and set out to ensure that result is obtained even though there isn't a credible narrative.

"In my experience, it would mainly be who I regard as career cops, ones who are really keen to move forward, and they take shortcuts. There are policemen like that on the force today. There were back then and there probably always will be.

"Everyone needs to be vigilant. And part of the problem is that the courts are pro-prosecution, so it makes it very difficult. You also often have clients who are scared ... Ngamu was the same. He really could have done something.

"When you have a case like that where the commissioner lets it go [and] they get away with it, that encourages them the next time"

A few years after the Ngamu case, Mr Hughes led the inquiry into the 1990 murders of Swedish tourists Heidi Paakkonen and Urban Hoglin.

The inquiry team was accused of making improper inducements to prisoners who testified that David Tamihere (serving life imprisonment for their murders) admitted the killings in jailhouse confessions, and of cynically manipulating identification evidence by parading Tamihere before media at a court appearance.

Two trampers who reported seeing a man with a blonde woman near Crosbies Clearing in the Coromandel had failed to identify Tamihere as the person they saw when police showed them a photograph. But after the court appearance, one of the two trampers identified him.

The defence team was also concerned by a police claim that in a second search of a small one-room shepherd's hut near Crosbies Clearing, they had found a tent with blood stains.

An unsuccessful appeal was launched after Hoglin's body was found in 1991 far from where police alleged Tamihere had killed the couple.

Hoglin's watch, which detectives claimed Tamihere gave to his son, was still on the body.

After he retired from the police, Mr Hughes was accused of impersonating a police officer and illegally detaining a British man in a 1995 incident that arose out of a financial dispute between honorary Dutch consul Alex van Heeren and a former business partner.

Mr Hughes was working as a private investigator for Mr van Heeren. The man he was accused of detaining, Bryan Cooper, was working for the former business partner.

Mr Hart represented Mr Cooper, who claimed he was detained and interrogated by Mr Hughes in the offices of a city law firm.

Two off-duty officers involved - one of them Mr Hughes' son - were disciplined under police regulations for taking on secondary employment without permission.

The allegations against Mr Hughes, who died in 2006, were investigated by a former colleague, George Wood (an inspector who later became North Shore mayor).

Superintendent Norm Stanhope, who was Auckland's police commander, said at the time that it was unnecessary to assign a senior detective from another district.

"The matter will be scrutinised by myself, and while I know John and have known John for a number of years, you can be assured that I would approach it in a totally objective manner and I would base a decision on the facts put before me."

No charges were laid.