Under questioning from Labor, Turnbull told parliament he had referred the matter to the secretary of his department, Martin Parkinson.

Just after Christmas, Cities Minister Jamie Briggs was forced to quit the ministry for inappropriate behaviour when overseas. Special Minister of State Mal Brough stood aside while police investigate his role in the Ashby affair.

Robert, assistant defence minister at the time, went to China in 2014, where he attended a signing ceremony between Chinese company Minmetals Exploration and Development and Australia’s Nimrod Resources. The agreement was related to a joint venture for mineral exploration in outback NSW.

The Herald Sun reported that Robert had previously said Nimrod Resources executive chairman Paul Marks was a “close personal friend”; Robert also said he had bought shares in two of Marks' companies. It also reported Marks has donated A$2 million to the Liberals in the past two financial years.

Robert’s office said he had been on approved leave and attended the ceremony in a private capacity.

But a statement issued by Minmetals Corporation in August 2014 about the ceremony said: “On behalf of the Department of Defence, assistant minister Robert extended his congratulations on the co-operation between Nimrod and Minmetals … assistant minister Robert also presented to Chairman Zhou a medal bestowed to him by Australian prime minister in honour of remembrance and blessing”.

Robert disputed the accuracy of the press release in his office’s comments to the Herald Sun, saying he had not presented a “medal” but had exchanged lapel pins with party official Zhongshu Zhou.

The statement of ministerial standards says: “A minister shall not act 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.”

Opposition Leader Bill Shorten asked Turnbull whether he would ask Robert what he had nominated on the official Chinese application form as the purpose of his visit – the categories include official visit, tourism, non-business visit, business and trade or work. Shorten asked Turnbull to report back at the earliest possible moment.

Turnbull indicated that Parkinson would look into this question.

“I deal with these matters very thoroughly and very seriously and in accordance with the code. That’s what will be done,” Turnbull said.

Late on Monday, Turnbull’s office released a letter from Robert to the Prime Minister, sent Monday morning in which he said that certain questions had arisen about his 2014 visit. “I am confident that I have not acted inappropriately but I am cognisant of the high standards expected of all ministers and I write to ask you to seek advice from the secretary of the Department of Prime Minister and Cabinet on these matters”. He said he would “fully assist” the secretary.

Shadow attorney-general Mark Dreyfus said Robert’s conduct seemed to be “the clearest possible breach of the ministerial code of conduct”. He said Robert and Turnbull needed to answer a number of questions as soon as possible.

“We need to know, for example, was the Minister for Defence or the then Prime Minister or the Department of Defence informed about this trip? Did the ambassador in Beijing or our embassy in Beijing extend consular assistance or the ordinary assistance they would to a visiting minister, to Mr Robert on this occasion? Did he go on a tourist visa or did he go saying this was for business purposes? Who paid for the trip? Who paid for his accommodation?” he said.

Michelle Grattan does not work for, consult, own shares in or receive funding from any company or organization that would benefit from this article, and has disclosed no relevant affiliations beyond the academic appointment above.