Federal police have launched a formal investigation into the dealings former Turnbull government minister Stuart Robert had with a Chinese mining company.

“The matter is now being investigated and while this occurs, it is not appropriate to provide further comment,” an Australian Federal Police spokeswoman said on Thursday.

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Mr Robert resigned from the frontbench after it was determined he had breached ministerial standards in taking a personal trip to China in 2014 that involved witnessing the signing of a mining deal and discussions with government officials.

Labor says the investigation shows there is a wider problem with proper standards of conduct in the government.

“We now have the situation that four Liberal federal members of parliament are involved in current investigations by the Australian Federal Police,” shadow attorney-general Mark Dreyfus told reporters.