Peter Costello has resigned as chairman of Nine Entertainment “effective immediately” days after the former federal treasurer was accused of assaulting a News Corp journalist at Canberra airport.

Costello said in a statement on Sunday evening: “The board has been supportive through the events of the last month and last few days in particular. But going forward, I think they need a new chair to unite them around a fresh vision and someone with the energy to lead to that vision for the next decade.”

Costello, who also quit as a director of Nine, was last week accused of pushing over a journalist from the Australian newspaper as he refused to answer questions about his embattled media company.

Costello dismissed the allegations saying “there was no assault” and that the journalist, Liam Mendes, “fell over an advertising placard”.

Costello was approached at the airport by Mendes on Thursday. The journalist began asking questions about Nine’s response to allegations of sexual harassment and bullying at the company.

A Nine spokesperson last week denied the company’s chairman struck the journalist.

“After arriving at Canberra airport, the chairman [Costello] was confronted by a journalist from the Australian. In the course of filming the chairman while walking backwards, the journalist collided with an advertising placard and fell,” the spokesperson said at the time.

“At no point did the chairman strike the journalist.”

A spokesperson for ACT policing said on Sunday evening they had not received any reports regarding the airport incident.

The former deputy chair Catherine West has been appointed the new chair of Nine, the company said in a statement.

“Last year the board engaged a search firm to identify potential new directors as part of renewal and succession plans. The board’s nominations committee will take this process forward.

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“The board wishes to thank Mr Costello for his contribution to Nine over more than a decade including eight years as chairman. Mr Costello played a key role in Nine milestones such as its successful re-listing on the Australian Stock Exchange in 2013, securing the landmark News Media Bargaining Code and the transformative merger with Fairfax Media.”

Costello said he had previously flagged retiring from the board after the July Olympics and before November’s annual general meeting but had pulled forward that timing.

The former news and current affairs director Darren Wick abruptly left Nine in March after 29 years with the company after complaints of sexual harassment and inappropriate behaviour. Wick has not commented publicly.

Nine’s chief executive, Mike Sneesby, has come under pressure about what he knew of the allegations against Wick and the terms of Wick’s departure, believed to include a payout close to $1m, most of which is entitlements.

Costello said on Sunday: “Mr Sneesby has always had my full support as CEO. The company has set up a robust process to investigate historical complaints which has my full support. I believe it will get to the bottom of any unknown issues.”

The media union MEAA said on Sunday that Nine had been too slow to acknowledge the concerns raised by editorial staff about management’s failure to provide safe workplaces.

“Poor management culture starts at the top and the announcement of a new chair is an opportunity to reset by beginning to take the actions that the previous chair and senior management have been unwilling or unable to do,” the union’s acting director, Michelle Rae, said.