The "Mr Big" operation used by New Zealand Police to obtain evidence which helped secure the murder convictions of Kamal Reddy is also known as the "Canadian technique" because it was developed by the Royal Canadian Mounted Police in the 1990s.

Used in New Zealand and Australia at times, South Africa and apparently the Netherlands, the technique has not been successfully implemented in the United States, Hong Kong and United Kingdom.

Officially known as the "Crime Scenario Undercover Technique" in New Zealand, the modus was described by the majority in the Canadian Supreme Court in R v Hart [2014] SCC 52, [2014] 2 SCR 544 as follows:

"[1] When conventional investigations fail to solve serious crimes, police forces in Canada have sometimes used the 'Mr Big' technique.  A Mr Big operation begins with undercover officers luring their suspect into a fictitious criminal organization of their own making.  Over the next several weeks or months, the suspect is befriended by the undercover officers.  He is shown that working with the organization provides a pathway to financial rewards and close friendships.  There is only one catch.  The crime boss — known colloquially as 'Mr Big' — must approve the suspect's membership in the criminal organization. 

"[2] The operation culminates with an interview-like meeting between the suspect and Mr Big.  During the interview, Mr Big brings up the crime the police are investigating and questions the suspect about it.  Denials of guilt are dismissed, and Mr Big presses the suspect for a confession.  As Mr Big's questioning continues, it becomes clear to the suspect that by confessing to the crime, the big prize — acceptance into the organization — awaits.  If the suspect does confess, the fiction soon unravels and the suspect is arrested and charged."

According to the news reports which have emerged, the New Zealand Mr Big operation followed this scenario. Kamal Reddy has been sentenced to life imprisonment with a minimum non-parole period of 21 years.

The New Zealand Police Association has expressed its support for the technique for gathering evidence, although it believes the details should not be made public to prevent criminals from becoming aware. Other commentators have noted that it is only used for serious offending, but question the reliability of evidence thus gained. Publication of the ways in which the Mr Big operation was managed in the Kamal Reddy case became possible after Justice Asher lifted interim suppression orders and declined to make any suppression orders for the details of the Mr Big operation.

In 2007 (Tofilau v R), Australia's Chief Justice Gleeson felt that Mr Big operations would have a "limited life expectancy", assuming that the criminal community would read the law reports or media reports on the tactic. That doesn't seem to have happened in the jurisdictions where the technique is used. It has been used over 350 times in Canada.

While Canada was the home of "Mr Big", the Supreme Court decision in Hart established a two-pronged response for testing the admissibility of Mr Big evidence, and held that such evidence is inadmissible unless the prosecution can establish that admissions obtained are reliable because of the circumstances of the case.

The High Court of Australia reviewed cases from Victoria involving Mr Big evidence, in Tofilau v R [2007] HCA 39, (2007) 231 CLR 396. The law relating to evidence has since changed in Victoria. Six of the seven High Court justices (with Kirby J dissenting) decided that confessions obtained by Mr Big techniques were admissible, as long as they were made voluntarily with no compulsion from any source. Safeguards were also recognised in the need for the Police to record all interactions with the accused, and also in the assumption that the existence of any corroborating evidence was critical.

According to commentator Caroline Law, no Mr Big confession cases "have been found on American soil". The English police force apparently does not use Mr Big investigative techniques.

Discussion of the law around Mr Big in New Zealand is restricted because of suppression orders which are still in force. However, Supreme Court Justice Susan Glazebrook delivered a paper entitled "Mr Big Operations: Innovative Investigative Technique or Threat to Justice?"  to the Judicial Colloquium 2015 in Hong Kong in September 2015.

Dame Susan noted that the admissibility of confessions in New Zealand is governed by the Evidence Act 2006. 

"Statements made by a defendant that are proffered by the prosecution are admissible against that defendant unless excluded on reliability or oppression grounds. Statements may also be excluded because of a breach of the New Zealand Bill of Rights Act 1990 or other enactment or rule of law, or because they are unfairly obtained."

Dame Susan said the Court of Appeal decisions R v Cameron [2007] NZCA 564 and [2009] NZCA 87 upheld the decision to admit confessions arising from the Mr Big technique "on the basis that the confessions passed the reliability threshold and were not unfairly obtained, although in that case the lawfulness of the technique had been conceded."