An Australian judge who accused police of planting evidence against Arthur Allan Thomas threatened to drop his inquiry unless the New Zealand Government guaranteed he would not be sued for his comments.

Justice Robert Taylor headed a 1980 royal commission into Thomas' overturned conviction for the double murder of Harvey and Jeanette Crewe a decade earlier.

His report said Detective Inspector Bruce Hutton and Detective Len Johnston buried a used shellcase from Thomas' rifle in the Crewes' garden to frame the 32-year-old local farmer for the crime. The commission described the police action as "an unspeakable outrage".

But a month before the report was made public, the Sydney-based retired judge warned that he was not prepared to say the two officers fabricated evidence unless he was protected from defamation.

According to a phone conversation transcript obtained by the Weekend Herald, Justice Taylor angrily insisted on the same status as New Zealand judges, who cannot be sued for their comments.

"I won't give you a report unless I get this protection," he told then Justice Minister Jim McLay in notes reportedly made at the time.

"You want me to say the truth about Hutton. I will say it but I won't say it and be sued for it."

Later in the transcript Justice Taylor says: "Do you understand the lengths I am prepared to go to?

"I will go back to Australia and won't return again unless I get this protection. And I will not issue this report.

"I won't put myself at risk over this. I won't come back here and defend [legal] actions. I owe this country nothing."

Justice Taylor died in 1994. Mr McLay, who is now New Zealand's ambassador to the United Nations, confirmed the conversation took place.

"At one stage he threatened that he wouldn't continue with his work unless he had the protection," he said from New York.

"We had what you might call a bluff and counter-bluff discussion on the matter ... At one point he was all set to go home."

Mr McLay said he was not willing to change the law as Justice Taylor had insisted but he understood the judge's concerns, because the police union had already taken the royal commission to the High Court.

He decided on the "elegant solution" of tabling the commission's report in Parliament, which gave it full parliamentary privilege and therefore protection against defamation claims.

As Attorney-General, Mr McLay had recommended the pardon Thomas was granted a year earlier.

He compared the case to the Dreyfus affair in which a French officer of Jewish descent was falsely accused in 1894 of spying for Germany and later framed with faked documents. The political scandal divided France for two decades.

Mr McLay said many French people opposed re-opening the case because they were more concerned about protecting the social order than whether Dreyfus was innocent. He believed the Crewe murders divided New Zealand in a similar way.

"A lot of people saw attempts to clear Thomas' name as an attack on the system."

Police have consistently opposed the commission's findings.

Mr Hutton yesterday pointed to a 1981 finding by Solicitor-General Paul Neazor that there was no evidence to lay charges against him. The previously unreleased report said the case against the police depended on one civilian witness who said police had originally searched the Crewes' garden thoroughly, whereas four police said they had not.

It also relied on the evidence of two neighbours, who remembered hearing rifle shots at the property and seeing Hutton and a colleague driving away soon afterwards. Both were unclear about the date.