The finance minister has been crucial in getting legislation through the upper house, where the government needs the Greens or three of the five crossbenchers if Labor opposes bills.

Senator Cormann announced at the weekend he would retire from politics at the end of the year, after helping finalise the July 23 economic statement, the budget in October, and the half-yearly budget update in December.

Centre Alliance senator Rex Patrick said it would be difficult to replace Senator Cormann's three vital roles - leader of the government in the Senate, finance minister and cross bench negotiator - with one person.

"I can't think of anyone in the Senate that immediately springs to mind of being able to fill all those things that Mathias did," he told AAP on Tuesday.

He says he always found the government's Senate leader trustworthy and not one to take disagreements personally.

Plus, Senator Cormann's senior standing in cabinet gave him wide authority when it came to negotiations.

"If something comes off the rails in relation to what you've negotiated, he always had the ability to step in and go over those details, even if it was another minister's portfolio," Senator Patrick said.

"Those things are important, to go into a negotiation, have a free-ranging and confidential discussion and walk away disagreeing or not having met agreement and then moving on to the next thing."

Labor's Senate leader Penny Wong, who often sparred with Senator Cormann in parliament, described him as "a formidable opponent and a trusted counterpart" - and a friend.

Greens leader Adam Bandt called Senator Cormann a strong adversary but someone you could always talk to.

However, he said whoever took over the role would be handed a "poisoned chalice" after changes made in June to limit the number of motions minor parties could put up for debate.

"If the new leader wants the Senate to function smoothly, the Liberal-Labor pact to curtail crossbenchers' rights will be a major obstacle," Mr Bandt told AAP.

At the moment, manager of government business Anne Ruston and junior financial services minister Jane Hume help with crossbench negotiations, but insiders say they'll be hindered without the finance minister's gravitas and seniority.

Rumoured finance portfolio replacements are Trade Minister Simon Birmingham and Home Affairs Minister Peter Dutton.

The latter has also been linked to a move to the defence portfolio, assuming he wants to leave his powerful current department.

A wider reshuffle could be triggered if Immigration Minister David Coleman, who has been on personal leave since late 2019, also departs.

Australian Associated Press