Nine board chairman Peter Costello resigns after reportedly ...

Nine Entertainment board chairman Peter Costello has resigned, following reports he allegedly assaulted a journalist from The Australian at Canberra Airport. 

Nine confirmed the resignation in a statement, saying Mr Costello, a former federal treasurer, will "step down from the board effective immediately". 

Footage posted on The Australian's website on Thursday appeared to show reporter Liam Mendes approaching Mr Costello before asking him questions about a harassment scandal at Nine. 

The footage appeared to show Mr Costello walking towards Mr Mendes, before Mr Mendes falls backwards.

Answering reporter questions at Parliament House on Thursday evening, Mr Costello denied assaulting Mr Mendes.

Mr Costello said Mr Mendes was walking backwards while filming and tripped over an advertising placard.

In a statement, Mr Costello said he had flagged retiring from the board some time after the Olympics in July and by November at the latest. But today he informed the board he had pulled forward the timing and was standing down.

"The board has been supportive through the events of the last month and last few days in particular," he said.

"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."

Mr Mendes had been asking Mr Costello about claims of “alleged inappropriate behaviour” at the network, including by former senior news boss Darren Wick. 

Mr Costello was also questioned about claims that Nine chief executive Mike Sneesby asked staff to sign non-disclosure agreements after they made a complaint.

Mr Sneesby has denied that allegation. 

In the statement announcing his resignation, Mr Costello added: "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."

Catherine West was appointed chair of Nine Entertainment Company, Nine announced in a statement to the ASX on Sunday.

The statement said Ms West had worked in the media and entertainment industry for more than 30 years in the UK, Europe and Australia.

Mr Costello is Australia's longest serving treasurer, having held the role in John Howard's Coalition government from March 1996 to December 2007.

He retired as the federal Member for Higgins in 2009 after serving seven terms.