The man Prime Minister Tony Abbott cited to defend the embattled head of the royal commission into union corruption actually thinks Dyson Heydon should stand aside.

Mr Abbott on Friday relied on comments from one of the Coalition's most strident critics, barrister and human rights advocate Julian Burnside, that Mr Heydon was "an honourable person" to deflect criticism of Mr Hedyon's decision to speak at a Liberal Party fundraiser.

Mr Burnside made the observation on Thursday evening when stating that Mr Heydon, a retired High Court judge, would never have knowingly agreed to speak at a party fundraiser.