The resignation will give Prime Minister Malcolm Turnbull another position to fill as he considers a ministerial reshuffle during the weekend.
Robert has been under investigation over what he described as a “private” trip he took to China with Liberal Party donor Paul Marks, during which he attended the signing of a mining deal and met a Chinese vice-minister.
The trip was taken with the approval of former prime minister Tony Abbott.
A senior DFAT official told a Senate hearing on Thursday Chinese officials thought the then-junior defence minister was acting in an official capacity at the signing ceremony in Beijing.
The investigation by Department of Prime Minister and Cabinet secretary Martin Parkinson found Robert’s actions in making the trip to China were “inconsistent” with ministerial standards, the Herald Sun reported on Friday.
The investigation report was expected to be released on Friday.
Conservative ministerial colleagues have defended Robert, exposing a rift within the government.
Nationals leader Barnaby Joyce said he could not see any wrongdoing.
“What we have here is allegations he met people,” Joyce said.
“If meeting people in China is a crime then every politician in this building is gone.”
Treasurer Scott Morrison earlier in the week said it was a “ridiculous beat-up” by the Labor opposition and media.
Media reports suggest Morrison, a close friend of the minister, tried to save Robert’s job when the issue was considered at a meeting on Thursday night.
Labor frontbencher Chris Bowen said there was “open warfare” between Turnbull and some of his front bench.
“What we’ve seen is Scott Morrison going into bat for internal factional reasons, trying to save the career of a minister who had clearly, in an open-and-shut manner, breached the ministerial code of conduct,” Bowen told reporters in Sydney.
Labor argued the minister breached standards banning him from acting as “a consultant or adviser to any company, business, or other interests, whether paid or unpaid, or provide assistance to any such body, except as may be appropriate in their official capacity as minister”.
Joyce was the first government figure to publicly confirm the resignation.
“Once the details become apparent, you’ve just got to say sorry, goodnight Irene,” he told radio 2GB on Friday.
