Union whistleblower Kathy Jackson admits "we all make mistakes" in life - including a "charity shag" she had with Health Services Union barrister Mark Irving, SC, more than 20 years ago.

Speaking outside the royal commission into trade union corruption on Friday, Ms Jackson said: "Forget the former lover stuff. Everybody makes mistakes and has a charity shag along the way.

Alleged affair: Health Services Union barrister Mark Irving. Credit:Getty Images

"I just could not believe he had the audacity to sit there and want to cross-examine me."

Ms Jackson, who is on sick leave as national secretary of the Health Services Union, side-tracked the royal commission into union corruption for more than an hour on Thursday after she asked for Mr Irving to be stopped from cross-examining her because she had sex with him 21 years ago.