An inquiry into two senior ministers' jobs after retiring from politics could attempt to haul Prime Minister Scott Morrison and the head of his department before a parliamentary committee.
Christopher Pyne, who was defence minister before the election, and ex-foreign minister Julie Bishop are under pressure about consulting roles they picked up after leaving federal parliament.
Mr Pyne has a new defence-focused role with professional services giant EY, while Ms Bishop has been appointed to the board of a foreign aid contractor.
Department of Prime Minister and Cabinet secretary Martin Parkinson reviewed the appointments and found ministerial rules were not broken.
But Dr Parkinson's investigation didn't satisfy Centre Alliance senator Rex Patrick, who on Monday won support in the upper house to establish an inquiry into the ministerial code of conduct.
The code says ministers must not lobby or have business meetings with politicians or public servants within 18 months of leaving parliament on matters they dealt with in their final 18 months in office.
Senator Patrick said he would try to make Mr Morrison and the department secretary give evidence to a hearing as part of the committee's look at ministerial standards.
Mr Pyne and Ms Bishop are also expected to give evidence to the inquiry.
Government Senate leader Mathias Cormann warned against making it impossible for former ministers to earn a living.
"Clearly there is no breach and there is no case for this inquiry," he told parliament.
Labor frontbencher Kristina Keneally said it was absurd to think the former ministers were not preparing for life after politics in their final days in office.
She said Mr Morrison would not be able to dismiss the issue with one of his favourite sayings.
"This is not something just inside the 'Canberra bubble'. These are the ethical standards you and your ministers vowed to uphold," Senator Keneally said.