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A former undercover police officer's claims he deliberately gave false evidence could result in convictions being quashed, compensation granted and lawsuits taken against the Crown, one legal expert says.

University of Auckland law expert Associate Professor Scott Optican said criminal convictions could be overturned, depending on the importance of the evidence given at trial by former officer Patrick O'Brien.

The now 59-year-old Mr O'Brien infiltrated the shady world of drug dealers in the 1970s, later becoming a star witness for the Crown.

But racked with guilt and a "dreadful secret" that has plagued him for the past 30 years, Mr O'Brien wrote to Chief Justice Dame Sian Elias admitting to perjury in the trials.

He claimed he lied every time he gave evidence while under oath and had been complicit in sending at least 150 people to prison.

Professor Optican - a former United States prosecutor - said recantations were not uncommon.

"But if a police officer gets up there and says, 'I did perjure myself', and has no motive to do it other than to come clean, then I would imagine an appellate court would give it some credibility - but I don't know the circumstances of his recantation."

He said in terms of civil compensation, ex-gratia payments were possible from the Crown if someone is able to prove themselves innocent and to have been wrongfully convicted.

"There may also be the possibility of a civil lawsuit by the defendant against the Crown on the basis of the police officer's intentional misconduct as an agent of the Crown," he said.

Wellington lawyer Bruce Squire, QC, who is investigating the allegations on behalf of the police, could not be contacted for comment last night.

But former undercover policeman turned Auckland criminal lawyer Tony Bouchier said the revelations weren't news to him, as Mr O'Brien had made the allegations on a television documentary, Going Under, which aired this year.

"I'm not sure if I'm surprised or not ... I don't know if it's as serious as what's alleged," said Mr Bouchier.

Asked if he thought there was any truth to Mr O'Brien's allegations, he said: "It's his experience, and I really can't say, but for me Paddy O'Brien is a man of his word and if he says it's happened, it's happened."

Mr Bouchier, who was in Sydney yesterday, said he was "not comfortable" speaking about the danger and long-term stress undercover police officers were exposed to.

But on the documentary he revealed he had lost close friends to the undercover programme and was "incredibly angry" the force had put him in danger of going "the other way". He said undercover agents he knew had gone to jail; others had beaten their wives or become alcoholics or drug addicts.

"The work itself was nearly the end of me," he said.

In the documentary, Mr O'Brien said he resigned from the force "because basically I was a corrupt policeman".

"Some of the things we did were totally unacceptable ... I had no right to be wearing that uniform."

In his letter he admitted to tampering with evidence and deceiving his operators, usually high-ranking detectives stationed in the communities where Mr O'Brien operated undercover.