Kathy's uncharitable end to the affair

Observers of the trade unions royal commission were treated to extraordinary details of union whistleblower Kathy Jackson's affair with a solicitor 21 years ago.

But overnight the eight-week fling shifted from grounds for perceived bias to mere "charity shag".

On Thursday Ms Jackson, the Health Services Union (HSU) national secretary, filed an affidavit detailing her affair with solicitor Mark Irving in the early 1990s, when she was married to her first husband.

The affidavit described how they had sex at Mr Irving's house after Friday night work drinks.

The house was "near the cemetery and had a barber's chair in the lounge".

Mr Irving is now the HSU's barrister - representing it in the commission and in other legal action brought by the union against Ms Jackson.

Ms Jackson sought a ruling that the historic encounters should disallow Mr Irving's cross-examination of her on grounds of his perceived bias after she ended the affair.

The application failed and Mr Irving proceeded to ask her about her alleged misuse of union funds.

On Friday Ms Jackson had downgraded the liaison.

"Forget the former lover stuff - everybody makes mistakes and has a charity shag along the way," she told reporters outside the commission.