The former federal treasurer quit the role after being accused by the reporter of assaulting him at Canberra’s airport on Thursday. Mr Costello denied the allegation later that day, claiming The Australian’s Liam Mendes fell after backing into an advertising placard while asking him questions.

“I thank the board for their support over the last decade and particularly during the events of the last few weeks,” Mr Costello said in a statement.

“There are enormous challenges ahead but I believe Nine is the best placed Australian media company to weather them and prosper.”

Current deputy chair Catherine West steps into Mr Costello’s former role and thanked him for his “dedication and commitment” to the company.

“As chairman, he has always put the needs of the company first and his decision to stand down and pass on the baton of leading Nine at this time is in line with that approach.”

Mr Costello had been chairman since 2016.

Ms West was appointed deputy chair in September 2023 after joining the board in 2016, according to Nine’s corporate website.

She has “more than 25 years of business and legal affairs experience” in the media industry including at Sky in the UK, her bio states.

In a video published by The Australian, Mendes follows Mr Costello through the airport as he asks about Nine chief executive Mike Sneesby.

“I walked past him, he walked back into an advertising placard and he fell over,” Mr Costello said.

“I did not strike him … if he’s upset about that, I’m sorry, but I did not strike him.”

Mr Costello appears to walk towards the reporter at one point before the journalist falls backwards.

“You have just assaulted me,” Mendes says in the video.

Nine has been embroiled in scandal across the last few weeks, dealing with sexual harassment allegations within the organisation.

Labor Treasurer Jim Chalmers criticised Mr Costello and said if anyone knew it was really important to treat journalists with respect, “it should be the chairman of a major media organisation”.