Prime Minister Robert Muldoon's backroom advisers worked to debunk the embarrassingly critical Erebus crash report even before it was released.

The revelations to the Herald on Sunday come on the eve of the 25th anniversary of the Antarctic crash that claimed 257 lives when an Air New Zealand DC10 flew into the slopes of Mt Erebus, on November 28, 1979.

The one-man commission, the late Justice Peter Mahon, was slammed by Muldoon who refused to table his 1981 report which accused Air New Zealand witnesses of participating in an "orchestrated litany of lies" on the witness stand.

An accident report had blamed pilot error - far less damaging to the Government than systems failure at the state-owned airline.

Justice Mahon found a navigation computer had been incorrectly changed so the plane was programmed to fly into the mountain, and that Air New Zealand witnesses had lied to cover up other mistakes that pointed blame at the carrier.

Muldoon responded with venom - the findings were potentially fatal to the Government-owned carrier - while Air New Zealand prepared an appeal against the lying accusations in court.

Nearly 23 years later, Muldoon's key advisers reveal officials were getting advice that would counter the report, months before it was released.

The prime ministerial advisory group had been leaked indications of where the Mahon inquiry was going, and hired aviation experts and even met Air New Zealand in a bid to explore their concerns.

"The Prime Minister's department in those days tended to work a bit in advance of events," said Muldoon's press secretary of the time, Brian Lockstone.

"It was at that point that it was recognised that perhaps there was going to be a report that might have been strong on emotion and perhaps some fixed ideas that might actually miss the greater point."

Pressure and conflicting opinions came from all sides, including multi-national companies who built the plane, he said.

The advisory group received a draft of the report reviewed by two pilots with polar and whiteout flying experience, working independently of each other.

"They came up with some very interesting conclusions that basically said that poor old Peter Mahon had got it wrong," he said.

Former head of the department Gerald Hensley said the advisers felt there were problems with Mahon's logic and told Muldoon who said they should have a closer look.

The consulted pilots argued that only one person flew the aircraft "and that's the pilot," he said.

"From all that we did have some differences with Justice Mahon's argument that the plane, in his phrase I think, 'was programmed to fly into the mountain from the moment it left [New Zealand]'."

Justice Mahon's widow Margarita was horrified Air New Zealand were in on the advisory group meetings.

"Muldoon wanted to make Air New Zealand into the seventh wonder of the world. Nothing, nothing was going to damage it. That was my understanding. They had no right to be there. No one should have had any knowledge of what was in that report at all."