Malcolm Turnbull's office has rejected claims Barnaby Joyce breached the ministerial code of conduct when his lover was employed by fellow MPs, arguing Vikki Campion could not be considered the Deputy Prime Minister's "partner" at the time.

Ms Campion left Mr Joyce's employ in April last year when their affair - which ultimately ended his 24-year marriage - began creating tensions in the office.

The 33-year-old was then given a new well-paid role with Resources Minister Matt Canavan, a close ally and fellow former staffer of Mr Joyce. Reports have suggested the job carried a salary of about $190,000 a year. When Senator Canavan stood aside over dual citizenship doubts in 2017, Ms Campion then moved to the office of then-chief Nationals whip Damian Drum.

But the ministerial code of conduct makes it clear partners cannot be employed by any other minister without an explicit prime ministerial go-ahead.