Defence Minister Linda Reynolds has been admitted to hospital and is taking medical leave.
The minister was due to give a speech at the National Press Club in Canberra on Wednesday.
Senator Reynolds has been under pressure following rape allegations made by former staffer Brittany Higgins against a colleague.
Senator Reynolds was taken to hospital following advice from her cardiologist relating to a pre-existing medical condition.
The prime minister has called Senator Reynolds to express his concern and sympathy, and to wish her a speedy recovery.
Foreign Affairs Minister Marise Payne will act in the role until Senator Reynolds returns from leave.
Ms Higgins tweeted following news of Senator Reynolds's hospitalisation.
"I genuinely hope [Linda Reynolds] is okay and wish her all the best with her recovery," she wrote.
"Let’s just hope that from this whole horrible situation there will actually be some fundamental reform to the MOP(S) Act for vulnerable staff and improvements to the workplace culture in Parliament House."
I genuinely hope @lindareynoldswa is okay and wish her all the best with her recovery. (1/2)
— Brittany Higgins (@BrittHiggins_) February 23, 2021
Brittany Higgins' rape allegations
Last week Ms Higgins made public the allegations she was raped at Parliament House by a colleague back in 2019.
The federal government has come under intense scrutiny over its handling of the allegations.
Ms Higgins, who was 24 at the time of the alleged incident, said she would be filing a formal complaint with the Australian Federal Police.
"I have re-engaged with Australian Federal Police and will proceed with a formal complaint regarding the crime committed against me in what should be the safest building in Australia," she said in a statement on Friday.
"The Australian Federal Police have made assurances to me that they will handle this matter thoroughly and transparently."
Prime Minister Scott Morrison has been forced repeatedly to clarify when he learned of the allegations made by Ms Higgins.
Mr Morrison told reporters last Tuesday he only found out about the incident on the Monday morning before Ms Higgins spoke out.
He denied there was a "don't ask, don't tell" policy in his office.
"I can assure you there is no such culture and I'm not happy about the fact it was not brought to my attention," Mr Morrison said.
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