Defence Minister Linda Reynolds has been admitted to Canberra Hospital as a precautionary measure and will take indefinite medical leave.
The step was taken following advice from her cardiologist relating to a pre-existing medical condition, Senator Reynolds said in a statement.
The minister had been due to give a National Press Club address later Wednesday on security matters, but was expected to face a grilling over the treatment of her former staffer Brittany Higgins after she reported an alleged sexual assault in the minister's office.
Prime Minister Scott Morrison phoned Senator Reynolds on Wednesday to express his concern and sympathy, and to wish her a quick recovery.
Foreign Minister and former defence minister Marise Payne will act in the Defence role until Senator Reynolds returns from leave.
Health Minister Greg Hunt told reporters that Senator Reynolds had wanted to proceed with the NPC address today, and alluded to the pressures that the political arena of parliament has on individuals.
"We have to remind ourselves that this is the most intense, arguably, environment in Australia," Mr Hunt said. "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 so reaching out and she is a good person and so she needs our support as she has our support."
Senator Reynolds was visibly upset while answering questions in parliament following revelations of Ms Higgins complaint and subsequent departure from her work as a media advisor for the government.
The senator has apologised to Ms Higgins, and deflected questions on actions taken after the allegation was reported as not being her story to tell. Mr Morrison revealed that Senator Reynolds met an AFP assistant commissioner after learning of the allegations.
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