Senator Reynolds went on sick leave two weeks ago shortly after a former Liberal staffer Brittany Higgins alleged she had been sexually assaulted by a colleague in the minister's office two years ago.

The case is now in the hands of the police.

In a statement on Sunday, Senator Reynolds will extend her leave until April 2 after being re-assessed by her cardiologist in relation to a pre-existing medical condition.

Senator Reynolds has advised Scott Morrison of her doctor's assessment and will continue to consult with the prime minister as required.

Foreign Minister Marise Payne will continue to act as defence minister in her absence.

But Labor's home affairs spokeswoman Kristina Keneally weighed in, questioning how Senator Reynolds can pretend to be well enough to remain defence minister but not well enough to front up to questions in parliament.

The Senate sits for four days starting on March 15, which is followed by a week of gruelling Senate estimate hearings.

"To be defence minister, to be any minister, you need good judgement and sound character," Senator Keneally told reporters in Sydney.

"What we have seen from Linda Reynolds is that she has neither. She needs to go, her position is untenable."

Last week Senator Reynolds became embroiled in a row after it was reported she'd told her staff Ms Higgins was a "lying cow".

She has since apologised, but said the comments were not over the rape allegation.

But Senator Keneally said the minister cannot call an alleged victim of a rape in her own office a "lying cow' and think it has no consequences.

"She should make that decision today and she should go," Senator Keneally said.

"And if she won't, the Prime Minister Scott Morrison should show leadership."

Australian Associated Press