Channel Nine chairman Peter Costello has stepped down from the broadcaster days after he allegedly bumped into a journalist

Peter Costello has stepped down as chairman of Nine Entertainment Co. days after he allegedly bumped into a journalist at an airport who then fell on the ground.

Nine chief executive officer Mike Sneesby said in a statement released on Sunday afternoon: 'I want to thank Peter for his contribution to Nine over more than a decade'. 

'As the announcement makes clear, he has played an important role in the big Nine milestones over that time - a period marked by dramatic digital transformation for the media industry and our company.'

Mr Costello, who was the Federal Treasurer in the Howard government and once tipped as the next Prime Minister, was questioned by The Australian journalist Liam Mendes after arriving at Canberra Airport on Thursday afternoon.

Mr Mendes approached him and fired off a series of question but appeared to bump into Mr Costello mid-sentence, though the former Nine boss later said he had backed into a bollard.

Mr Costello will be replaced by Nine's deputy chairman Catherine West, a former Sky executive in the UK.

In the airport altercation Mr Costello, 66, ignored a series of questions from Mr Mendes about a $1million payout made to the channel's former news boss Darren Wick, who faces allegations of sexual harassment.

Mr Mendes introduced himself to Mr Costello and questioned why the Nine boss hadn't shown his support for the network's chief executive officer, Mike Sneesby.

Mr Sneesby has come under fire following reports he approved the large payout to Mr Wick.

Just a minute after Mr Mendes began firing off questions, he lost his balance and fell over.

GoPro footage appeared to show Mr Costello walk into Mr Mendes, knocking him to the ground.

A flustered Mr Mendes picked himself up and followed Mr Costello into the airport car park.

'Mr Costello, you can't do that, you can't do that, it's all captured on camera,' Mr Mendes said.

'Is that behaviour really appropriate to a journalist who's working?' Mr Mendes asked.

On Thursday evening outside Parliament House, Mr Costello denied he had knocked over the journalist.

'There is no assault,' Mr Costello said.

'I did not lay a finger or a fist or anything else on him.'

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

'I have seen it happen a million times … reporters back into the bollards and fall over,' he said.

'I did not strike him.'

Mr Costello also denied that viewers of the footage would describe him as being aggressive.

Before Mr Costello appeared to push Mr Mendes over he had asked the Nine Chairman: 'Do you support Mr Sneesby as CEO and his handling of the Darren Wick saga?'

Mr Wick, the former head of Nine News, was the subject of serious misconduct accusations shortly prior to his resignation from the company in March.

It's understood both Mr Costello and Mr Sneesby were aware of the allegations at the time of Mr Wick's resignation.

Mr Sneesby has since faced criticism for approving Mr Wick's $1million financial settlement, which included a payout and leave entitlements.

It's understood Mr Costello was unaware of the payout until after Mr Sneesby approved it and has not since publicly supported the CEO.

Mr Costello signed a letter to all Nine staff last Thursday, obtained by the ABC, which acknowledged the 'serious' claims against Mr Wick. 

'We recognise we need to do more,' the letter read.

All Nine staff will be required to complete refreshed sexual harassment prevention training by June 30.

It's understood Mr Costello was in Canberra on Thursday for the opening of Nine's new office in Parliament House.