The ABC's acting chair says it's an "unreasonable expectation" for the public to know why Michelle Guthrie was sacked as managing director of the national broadcaster.

Dr Kirstin Ferguson was named on Friday as the broadcaster's acting chair after Justin Milne quit.

Mr Milne's resignation on Thursday came after the board asked him to step aside following media reports in the wake of Ms Guthrie's sudden sacking halfway through her term.

Dr Ferguson, who has been on the board since 2015, declined to say when allegations of Mr Milne demanding the sacking of two senior reporters first arose.

She also rejected suggestions the public were owed an explanation for the departure of Ms Guthrie.

"I think it is an unreasonable expectation. It is unfair on the person involved and I just won't go there," she told ABC TV on Friday.

She said it was time to knuckle down and focus on the future of the ABC.

Acting Labor leader Tanya Plibersek said there was a cloud over the entire ABC board because they only asked Mr Milne to step down after allegations against him became public.

"I think other board members really do need to examine whether they have fully discharged their responsibilities to protect and defend the ABC," Ms Plibersek said.

"Or whether, in fact, they've been voluntarily or involuntarily complicit in the government's political interference."

Mr Milne has denied demanding any journalist be sacked but maintains he was entitled to intervene on issues of editorial accuracy and independence.

Labor has called for the appointment of a permanent chair to be a bipartisan process and will join the Greens in trying to initiate a Senate inquiry.

Communications Department secretary Mike Mrdak is also looking into issues around the ABC board.