Defence Minister Linda Reynolds has taken medical leave and been admitted to hospital as a "precautionary measure".
Senator Reynolds was due to address the National Press Club today, where she was expected to face questions about her handling of sexual allegations made by her former staffer, Brittany Higgins.
In a statement, Senator Reynolds said she was taking medical leave on the advice of her cardiologist, and that it related to a pre-existing medical condition.
The statement said Prime Minister Scott Morrison has phoned Ms Reynolds to "express his concern and sympathy".
It comes ahead of former Liberal staffer Brittany Higgins's plans to make a formal complaint to police today, alleging a male colleague raped her inside Senator Reynolds's ministerial office in 2019.
Senator Reynolds has faced sustained pressure over her handling of the alleged rape complaint.
Foreign Affairs Minister Marise Payne will act as Defence Minister until Senator Reynolds returns to work.
Health Minister Greg Hunt told reporters at Parliament House that Ms Reynolds had taken the leave reluctantly.
"Linda was desperate to appear before the National Press Club today, desperate to appear," he said.
"It was only on the strongest medical advice that she took the reluctant decision not to do it."
Senator Reynolds was the defence industry minister when the alleged rape occurred.
Mr Morrison promoted her to Cabinet and the senior defence portfolio after the 2019 election.
She has faced internal pressure within the Coalition in the last week, with some MPs and Senators telling the ABC they thought she should resign or be sacked.
More to come.