In the space of a fortnight, the Heydon royal commission into trade unions has gone from predator to prey.

The roles began to be reversed when it was revealed that the commissioner, former High Court judge Dyson Heydon, had agreed to give a speech to a legal dinner which was described as a fund raiser for the Liberal party.

This led to the ACTU applying for Mr Heydon to stand down, arguing that the “reasonable hypothetical bystander” might believe that he would not bring a fair and impartial mind to the inquiry.

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The unions flagged that they did not expect their application to be successful, but Mr Heydon’s ruling has been delayed multiple times, fuelling speculation over the future of the inquiry.

The latest controversy turns on Mr Heydon’s explanation of when and why he told the organisers that he would not speak at the function.

ACTU Secretary Dave Oliver outside the commission. Photo: AAP

On August 13, Mr Heydon released a statement which said the event organisers were informed at 9.23am that day that he would not be able to give the speech, if there was any possibility it could be described as Liberal event.

However, The Australian newspaper reported on Thursday that a lawyer, former journalist and ex-Labor ministerial staffer, Marcus Priest, inquired about the planned speech with an official of the NSW Bar Association at about 5.30pm on August 12.

The Bar Association official reportedly sent Mr Priest details of the planned speech, but became concerned that the event might attract publicity.

The official is said to have emailed counsel assisting the commission, Jeremy Stoljar SC, at about 7pm that night, asking if Mr Heydon was aware the lecture was a Liberal party event. Mr Stoljar is said to have replied: “I’ll raise that with him.”

ACTU secretary Dave Oliver on Thursday raised concerns that the commission’s media release on August 13 said that Mr Heydon acted to withdraw from the event before it attracted any media attention.

“We believe there has been inadequate disclosure of relevant documents made by the Commission as to this matter,” Mr Oliver said.

On August 17, the ACTU asked for all relevant documents such as emails and were told they had been released.

But further material was released at another hearing on August 21, prompting counsel for the ACTU, Robert Newlinds SC, to complain that Mr Heydon had wrongly said that all documents had been released in the first place.

While this legal sparring was underway, there was no mention of the alleged tip-off to Mr Stoljar, whether he informed the commissioner, Mr Heydon, and the alleged email exchange with the official from the NSW Bar Association.

As Mr Stoljar noted last week, counsel assisting a royal commission is expected to play a neutral role in any application of apprehended bias.

Supporters of the government have already noted that the person who appears to have sparked the chain of events, Mr Priest, is a former staffer to two former Labor Attorneys-General.

However, it was Mr Heydon who initially accepted the invitation and who has since attempted to explain himself.

At the request of the ACTU, Mr Heydon’s ruling on whether he will stand aside has been delayed until Monday.

In the meantime, unions have demanded the release of “any emails or other communications which are referred to, or relate to, today’s article in The Australian.”

Suddenly, and unusually, it is the royal commission which has questions to answer.

The bottom line is that accepting the speech might have been an oversight, but it gave the unions and Labor a surprise opening to counterattack the commission and the Abbott government.

It will become clear in coming weeks whether this opening turns into a gaping hole.

Disclosure: The writer previously worked with Marcus Priest at The Australian Financial Review

Mark Skulley is a freelance journalist based in Melbourne. He was a reporter for The Australian Financial Review for almost 19 years, which included a decade covering national industrial relations and the world of work. View all of his columns here.