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.
However, she has been taken to hospital following advice from her cardiologist relating to a pre-existing medical condition.
Senator Reynolds has been under more than a week of intense scrutiny over her handling of rape allegations raised by former staffer Brittany Higgins.
Ms Higgins said she genuinely hoped the minister was okay and wished her all the best with her recovery.
“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,” she tweeted.
Parliamentary staff are employed under the act.
Health Minister Greg Hunt said Parliament House was arguably the most intense work environment in Australia as he addressed Senator Reynolds’ situation.
“I had some prior notification that she may have had an illness. I was not aware of the nature of it,” he told reporters.
“There are many intense environments and all of us need to be aware of the pressures and pains, the impact of each of us on each other, and reach out.
“She is a good person and so she needs our support as she has our support.”
Mr Hunt said the minister was desperate to appear at the press club.
“It was only on the strongest medical advice that she took the reluctant decision not to do it,” he said.
The prime minister has called Senator Reynolds to express his concern and sympathy, and to wish her a speedy recovery.
Foreign Minister Marise Payne will act in the role until Senator Reynolds returns.
Senator Payne was invited to replace Senator Reynolds at the National Press Club but declined.