There is no doubt about the credibility of mother-and-child killer Kamal Reddy's murder confession, a crown lawyer says, despite an undercover police operation used to bring him to justice.
Reddy's defence lawyer Jonathan Krebs has raised concerns about the so-called 'Mr Big' scenario, which he says could lead to false confessions.
Eight years after Pakeeza Yousef and her three-year-old daughter went missing, police had little to go on but they suspected Ms Yousef's ex-boyfriend Reddy.
Mr Krebs said that the operation started with undercover officers befriending the target before introducing him to a make-believe criminal organisation with corrupt contacts in the police.
"Eventually the suspect is made to realise that the real police are conducting an investigation into the person. At that point of course, the suspect thinks: 'Well, if I'm able to inveigle myself to this criminal organisation, they can make my problem go away'."
In Reddy's case, he eventually confessed to carrying out the murders and even led undercover officers to the site where he'd buried the bodies.
Crown solicitor for Manukau Natalie Walker prosecuted the Kamal Reddy case, where he was found guilty and sentenced to life in prison with a minimum non-parole period of 21 years.
Ms Walker said the technique was lawful and overseen by the courts.
"I suppose its true value can be seen in cold cases, such as the Kamal Reddy murder trial in Auckland earlier this year.
"In that case, as a result of the technique, Mr Reddy made full confessions to the murders, including providing detail as to how and where he committed them, and what he did to dispose of the bodies."
She said the technique led to the discovery of the burial site and that backed up Reddy's confession.
"The Crown said at Mr Reddy's trial, the reliability of his confessions was not in doubt."
The technique was also used in the Tawera Wichman case, who was last week jailed for three years and 10 months for the manslaughter of his baby daughter.
The legal argument over whether Wichman's confession should go before the jury went all the way to the Supreme Court, where three of the five judges ruled in favour.
Chief Justice Dame Sian Elias was one of the dissenting judges and she found the level of state deception, and lack of safeguards, unacceptable.
Police declined RNZ's interview request, but in a statement said the operation had resulted in convictions of violent offenders, who may otherwise have never been brought to justice.
"In cases such as the Reddy investigation, due to the significant time delay between the murders being committed and then reported... police were restricted in using more conventional avenues of enquiry, hence the use of undercover staff.
Police said undercover operations, such as the Mr Big scenario, were reserved for the most serious crimes, and were only used when they had exhausted other investigative options.
But Mr Krebs said the technique could result in false confessions.
"If such pressure is put on to a person to believe they are genuinely going to be charged for some sort of an offence but the organisation can make all this go away if they simply confess, then people may be forced to do so."
He said the scenario involved an extraordinary degree of state deception and the technique should be put on hold in New Zealand.
"There will be a lot of people out there who think: 'Well, if it solves crime, then let's just get on with it'. That's a slippery slope. The pendulum, in my view, has swung too far.
"We certainly want crime solved but we want to have it solved in a way that is consistent with our rights and what is consistent with a fair approach."