Then Australia Post boss Christine Holgate was stood down in October 2020 after she gifted expensive watches - valued at $20,000 - to four executives as a reward for their performance.

Mr Morrison, the prime minister, declared in parliament "she can go" if she didn't choose to stand aside voluntarily.

In 2021, a Senate inquiry heard evidence from the finance department not knowing Mr Morrison also held that portfolio in secret.

The Senate environment and communications committee, which held the review, was not told about Mr Morrison's role as co-minister.

Committee chair, Greens senator Sarah Hanson-Young said there were "serious questions" about whether Mr Morrison and the department had misled the parliament.

"Scott Morrison's treatment of Ms Holgate was appalling, and the committee found he had showed a lack of respect for due process and procedural fairness," she said.

"Now we find out he was the secret minister all along.

"I will be asking the committee to reconvene questioning the department and former ministers involved - including the 'minister in secret' Scott Morrison."

Senator Hanson-Young said she will be investigating whether a referral can be made to the privileges committee to consider whether the Senate was misled.

"The department either lied, withheld information, or didn't know," she said.

"This is a despicable state of affairs whichever way you look at it."

Mr Morrison told reporters last week the only decision he made in other portfolios was as resources minister in regard to the PEP-11 gas project off the NSW coast.

He took on the finance portfolio, without telling the then minister Mathias Cormann, on March 30, 2020, and failed to tell Senator Cormann's successor Simon Birmingham.

Australian Associated Press