There is speculation Senator Reynolds could be permanently replaced in the portfolio after she was admitted to hospital based on advice from her cardiologist.

She had been under intense pressure over her handling of the alleged rape of former staffer Brittany Higgins in 2019.

Senator Lambie, a former soldier, has long had grave concerns about the minister's ability to stand up to defence force top brass.

"Those men right now, right at the very top, probably need a boot fair up their clacker, mate," she told Sky News on Thursday.

The Tasmanian independent believes Mr Dutton is the only member of the government who could take on the job.

"His background in policing, he's a big guy, I don't think you'd see minister Dutton putting up with that sort of crap from those men," Senator Lambie said.

Prime Minister Scott Morrison rejected suggestions the defence minister's job was untenable after unnamed coalition MPs raised replacing her.

"Of course it is. Why wouldn't it be? She's doing a great job and I have great confidence in the work that she's doing," he told reporters.

Mr Morrison said her health was much improved but she would remain under observation for the next few days.

Mr Dutton also backed Senator Reynolds, who is a former army reserve brigadier.

"Linda has been under pressure over the course of the last couple of weeks, and she obviously has a pre-existing medical condition," he said.

"It's important for us to not speculate about what her job is and all of that. The priority is for Linda to get better and to be back at work as quickly as possible."

Senator Lambie said Senator Reynolds had mishandled Defence's response to the Brereton war crimes inquiry, a $90 billion submarine program and the alleged sexual assault.

"People have lost confidence in her and that's now turning into an issue for her," she said.

"Just for her own health situation at the moment maybe it would be better if she took a step down and just sit at the rear for a while."

Senator Lambie fears the minister has allowed powerful men in the military to run the Department of Defence.

"Something probably needs to be done," she said.

Senator Lambie joined politicians from all sides in wishing the minister a speedy recovery.

Foreign Minister Marise Payne is in charge of the defence portfolio while Senator Reynolds is on leave.

Australian Associated Press