Kamal Reddy had gotten away with murder until he became the target of a police undercover operation - the details of which can now be reported for the first time.
But initially police had little to go on. Reddy was careful. In December 2006, he strangled his ex girlfriend - Pakeeza Yusuf - with an iron cord and smothered her three year-old daughter Jojo with a pillow, leaving no blood in their Howick flat.
With the help of his uncle, he found a spot under a newly built motorway overbridge on Auckland's North Shore. He buried the bodies in the dead of night.
He then set about emptying their Howick flat of all their belongings, putting furniture in storage and dumping clothes in clothing bins around South Auckland.
Ms Yusuf had hardly any friends and a rift in the family meant few questions were asked.
Cold case heats up
Eight years after they were murdered, the bodies of the mother and daughter lay undiscovered and police had no leads.
But a six month-long undercover operation exposed Reddy as a murderer.
During the operation undercover police posed as members of a criminal organisation. They befriended Reddy and offered him a trial period, doing odd-jobs for small amounts of money, collecting debts and picking up stolen property.
Despite being apparent criminals, the values of the group - trust, honesty and loyalty - were repeatedly explained to him as he took part in jobs which he thought were real crimes.
He accompanied one of the group members to a meeting with a man who claimed to have carried out an aggravated robbery.
During the meeting, Reddy was told that the criminal organisation had corrupt contacts within the police who could make charges - and even evidence - disappear.
During the taped conversation, which was played to a closed court during Reddy's recent trial, the so-called robber repeatedly told Reddy how important it was to tell the truth, and to divulge every detail, no matter what. The fake robber eventually did just that and the criminal group made evidence "disappear", promising to supply the robber with a new identity and a fresh start.
In another scenario, a member of the criminal group was facing child sex charges. Reddy was told he revealed everything to the boss and, again, through the organisation's corrupt police contacts, that the charges disappeared.
Revealing the truth
After several other scenarios, including acting as a lookout in what he thought was the burglary of a firearms collector, Reddy was called in to meet with the boss.
He was hopeful he would be offered a job. But before that could happen, he was asked if there was anything in his past.
Initially Reddy said there was nothing, but when he was told as part of the undercover operation that the so-called criminal organisation had a corrupt police officer on its books, and that officer knew Reddy was a suspect in a murder, Reddy told them everything.
The confession was secretly filmed and it was later played to the court. In a calm voice, Reddy told the boss exactly how he had carried out the murders, who had helped and where the bodies were buried.
He even took an undercover officer to the very spot under the motorway overbridge where he had buried the bodies and posed for a photograph on the spot.
The operation was so convincing that when Reddy was arrested, he asked to call the man who he thought was his friend. The friend turned out to be one of the undercover police officers.
Taking down 'Mr Big'
Reddy's case was just the seventh time New Zealand police have used the technique, known as a 'Mr Big' operation. It was the second time it has gone before the jury and the first time it has been able to be reported in full.
The Crown and the police wanted the undercover operation suppressed to retain its efficacy. That was opposed by the media, including RNZ, on the grounds that trials should be held in public and the people of New Zealand had a right to know what the police were doing in their name.
Justice Asher agreed, saying the details of the case were important and allowed a meaningful and open discussion about the acceptability of the techniques.
The Crown said the double murder Reddy carried out would not have been solved if it were not for the technique and it appeared police had very little to go on, prior to his full confession.
But overseas, in Canada and Australia, the Mr Big operation has attracted controversy.
Watch: Edward Gay discuss the case on Checkpoint with John Campbell
The technique is not used in United States or the United Kingdom, where some commentators say it is illegal.
But in Canada it has been used at least 350 times. Some of the stings have taken up to three years, with one including 40 undercover officers.
The operation is not without controversy.
Rubin "Hurricane" Carter spent 20 years in prison for a murder he did not commit. Before his death in 2014 he worked for the Toronto based Innocence International and in 2009 told the Toronto Star that Mr Big stings could easily move from evidence gathering to entrapment.
He said he had no doubt that the Mr Big technique had sent real criminals to prison, but they had also resulted in wrongful convictions.
One of the Canadian cases is that of Kyle Unger who was convicted of a 1993 murder of a woman at a rock concert, partly on the basis of a Mr Big confession. During the confession, Unger spoke to officers about the scene of the crime, including a nearby bridge, but the bridge had not yet been built.
A Crown expert described hair found on the victim's body was similar to Unger's, but later DNA testing ruled Unger out.
He was freed from prison after serving nine years.
New Zealand Supreme Court judge Justice Glazebrook has written a paper on the technique, titled Mr Big Operations: Innovative Investigative Technique or Threat to Justice?
She summed up the debate about the technique like this:
"The debate takes place at a four-way intersection: between the private interests of victims and their families; the public interest in bringing offenders to justice; the private interest and right of suspects not to be coerced into confessions; and the public interest in guarding against wrongful confessions."
She ended her paper by saying courts had to balance those competing principles.