When trade union royal commissioner Dyson Heydon presides over a submission calling for his own resignation, he will be able to make an unbiased ruling, according to Professor Nicholas Cowdery QC.

Key points: The Australian Council of Trade Unions will lodge an application to disqualify Justice Dyson Heydon on Friday

The Australian Council of Trade Unions will lodge an application to disqualify Justice Dyson Heydon on Friday Justice Heydon himself will hear the application and decide whether he should stand down

Justice Heydon himself will hear the application and decide whether he should stand down Justice Heydon has been involved in a controversy over plans to speak at a Liberal party fundraiser

The Australian Council of Trade Unions (ACTU) will lodge an application on Friday to disqualify Justice Heydon from the royal commission into union corruption because of alleged Liberal bias.

That application will be heard by Justice Heydon himself, who will then have to decide whether he should stand down.

It has been revealed Justice Heydon initially agreed to appear at the Liberal-linked Sir Garfield Barwick lecture to be held in Sydney later this month.

He has since withdrawn from the engagement but conceded he overlooked the connection between the Liberal Party and the event.

Professor Cowdery, the former New South Wales director of public prosecutions, told Lateline that while a lay person might find it bizarre that Justice Heydon had to decide his own fate, it was normal legal process.

"I say that because you have to understand the nature of legal training, the nature of the legal mind after many long years of practice and the application of the law in that sort of environment," he said.

"This has been the process since the year dot when some challenge is made to the impartiality of a judicial officer or somebody sitting in the position of a commissioner.

"It's that person who makes the decision as to whether or not a reasonable lay person would have a reasonable apprehension that that person, the subject of the inquiry, could not bring an impartial mind to the inquiry."

Professor Cowdery said it was a trap to think that judges and commissioners were divorced from reality or the community.

"Judges see more of the slice of life in the community than ordinary individuals do in their daily lives. The community parades before them in the cases that come before them," he said.

"In something like a royal commission there's a huge range of people who come forward with a huge range of experiences and input into life and so forth.

"Judges and commissioners, inquirers of one sort or another, are in a very good position to make the sorts of judgments that are required."

Unions prepared to appeal to higher courts

The union movement's attempt to disqualify the royal commissioner for bias follows months of labelling the inquiry a "political witch hunt".

"The ACTU has always maintained that the royal commission is a political witch hunt by Tony Abbott designed to weaken his political opponents," secretary Dave Oliver said in a statement.

"When it came to light that commissioner Dyson Heydon had agreed to speak at a Liberal Party fundraiser, we called on Prime Minister Tony Abbott to shut down the royal commission.

"Given Tony Abbott has failed to act, the ACTU must now take further action."

If the move is unsuccessful, the unions will appeal to higher courts.

In Question Time on Wednesday Mr Abbott accused the Opposition of pursuing Justice Heydon because they had something to hide.

"And based on the evidence before the royal commission so far, there is a very great deal to hide," he said.

"[Dyson Heydon] is an honourable man. He should be allowed to get on with his job and members opposite should stop trying to protect and defend the indefensible."

Professor Cowdery did not think the revelations about the Sir Garfield Barwick address were grounds for Justice Heydon to disqualify himself.

However he said the royal commission would likely continue to be attacked politically.

"The problem is, of course, that any adverse findings against anybody remotely connected with a party other than the Liberal Party are going to be degraded and snubbed because of the events that have occurred."