Union officials who have appeared before the royal commission into union corruption, including Opposition Leader Bill Shorten, could take legal action against commissioner Dyson Heydon, on the basis of a perception of bias against them, an expert says.

At least one of the five unions that have appeared before the royal commission is considering civil action against Mr Heydon in the Federal Court.

Fairfax Media revealed on Thursday that Mr Heydon was billed as the keynote speaker for a Liberal Party fundraiser this month. He later said he was withdrawing from the function. Labor and senior union figures have decried the commission as a political witch-hunt since it began.

Patrick Keyzer, who's head of La Trobe University's law school, said he respected the former High Court judge, describing him as a "distinguished Australian jurist", but said there would be an obvious difficulty with a royal commissioner agreeing to speak at a fundraiser for the Liberal Party.