Special Minister of State Scott Ryan has defended former trade minister Andrew Robb, who took an $880,000 part-time job as a consultant to a Chinese billionaire days after the 2016 federal election.

"We have to be careful where someone has a broad portfolio — particularly someone like Andrew who was a senior businessman before he came into parliament — isn't prohibited completely from work after they leave public work," Senator Ryan said.

His comments come after a Four Corners-Fairfax investigation revealed ASIO warned political leaders that the Chinese Communist Party may be influencing the Australian political system through multi-million-dollar donations by influential businessmen.

One of those businessmen was property developer Huang Xiangmo, who along with associates donated $50,000 to Mr Robb's campaign financing vehicle, the Bayside Forum, on the day the Free Trade Agreement was signed in 2014.

In a separate development, the investigation found Mr Robb began working as a consultant to yet another Chinese billionaire, Ye Cheng, on July 1, 2016 — the day before the federal election.

He had previously announced he was resigning from the Melbourne seat of Goldstein and ceased to be an MP on May 9.

Mr Cheng's company, Landbridge Group, was embroiled in controversy when it was awarded a 99-year lease over the port of Darwin.

No claim Robb breached code: Ryan

Senator Ryan said the former trade minister's $880,000 part-time position with the Landbridge Group did not breach ministerial code of conduct rules, describing the situation as "a complex issue".

"There is no claim that Andrew Robb has in any way breached the code because it does prohibit dealing with officials that you dealt with as a minister, on issues that you dealt with as a minister, or on knowledge you had as a minister," Senator Ryan said.

"There has been no claim about that."

When asked whether Mr Robb's position was "appropriate", Deputy Prime Minister Barnaby Joyce said "that is really a question that Mr Robb should answer".

"I think it is only fair and reasonable that people are curious enough to want to know the answer and if further discussions need to be held in regards to people's roles, then I will always back in our nation," Mr Joyce said.

"I believe absolutely that there should never be any even implied undue influence."

Mr Huang also reneged on a $400,000 pledge to Labor in June last year, after its defence spokesman took a hard line on China's militarisation of the South China Sea.

The next day, Senator Sam Dastyari appeared with Mr Huang at a press conference exclusively for Chinese media, where he echoed Beijing's line on the disputed waters.

Senator Dastyari later lost his shadow cabinet position over revelations that Mr Huang and a second Chinese donor had paid for some of the Labor figure's expenses.