A top lawyer investigating claims that a former undercover policeman lied in court to wrongfully convict at least 150 people has finally obtained the 1970s court files - one year after the inquiry began.

Police hired Wellington lawyer Bruce Squire, QC, to investigate Patrick O'Brien's confession that he tampered with evidence and committed perjury by lying as a Crown witness in drugs trials in the 1970s.

Mr Squire was appointed last March and wrote to Mr O'Brien asking about the nature of his lies and the names of the defendants.

Police HQ denied Mr O'Brien's request for the names of his targets and Mr Squire was also unable to obtain the information until recently.

In a December email to Mr O'Brien, the lawyer wrote: "I have been endeavouring to obtain from the police and the courts, the details of the persons who were prosecuted as a result of your undercover police activities.

"When that information is to hand and assuming I am at liberty to disclose it, it may assist in identifying the persons who may have been affected by your claimed perjured evidence."

One year after being appointed to the inquiry, Mr Squire told the Weekend Herald he had finally obtained some of the court files in which Mr O'Brien said he lied.

"I'm having to try and locate files in excess of 30 years old. That carries its own problems," said Mr Squire. "I have some but not all of them. Some of the court records no longer exist, some cannot be found. We're dealing with matters that go back to 1974."

Police spokesman Jon Neilson said police had given Mr Squire some information, from historic records, about people who were convicted as a result of prosecutions in which Patrick O'Brien was involved.

Mr O'Brien is hitchhiking around a remote part of the South Island and is unable to communicate via email.

He wrote to Chief Justice Dame Sian Elias in November 2007 admitting to perjury, saying he was racked with guilt after carrying a "dreadful secret" for more than 30 years.

In the confession letter, Mr O'Brien said he could not guess the number of people with convictions or imprisoned "because of my lies", because he stopped counting arrests at 150, halfway through his three-year stint.

The shame and stress of the work broke Mr O'Brien, he said. He resigned from the police and fled New Zealand, "haunted, traumatised and scared".