Prime Minister Scott Morrison has written to the head of his department seeking advice on whether former colleagues Christopher Pyne and Julie Bishop have breached the ministerial code of conduct.

Former defence minister Christopher Pyne and ex-foreign minister Julie Bishop have come under fire for jobs they've taken since leaving politics.

Government Senate leader Mathias Cormann said Mr Morrison had written to Department of Prime Minister and Cabinet secretary Martin Parkinson for advice on how the ministerial code of conduct applies to all previous governments.

"As part of this he has sought advice on actions that can be taken to ensure compliance with the code," Senator Cormann told the Senate, making reference to former ministers in the Morrison government.

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Mr Cormann made the statement shortly before Centre Alliance Senator Rex Patrick was due to move a motion to establish an inquiry into Mr Pyne's new job and the ministerial rules governing post-politics jobs.

That's now on hold.

"Noting Senator Cormann's statement in relation to the ministerial standards, I seek leave to postpone this motion until the next sitting day," Senator Patrick advised the chamber.

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The code stipulates that 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.

However, the standards are rarely enforced.

Mr Pyne has a new defence-focused role with professional services giant EY, while Ms Bishop has been appointed to the board of an international aid contractor.

Nationals MP Barnaby Joyce said Mr Pyne's defence-related role was "certainly very questionable" but stopped short of supporting an inquiry.

"I think it's for him to answer for, others to answer for. I'm not going to say there's no question there, i think there's a question there that needs to be answered."