Porter said all he knew about the allegation was what he had read in the media, but the details of the claim had never been put to him.
He said he would not resign his ministerial position, but would take leave for a couple of weeks to “assess and hopefully improve” his mental health.
Police had said the allegation could not be investigated because the alleged victim did not make a formal statement to them detailing her account before she committed suicide last year.
Earlier this week, Prime Minister Scott Morrison said he had spoken to the then unnamed minister who had “vigorously rejected” the allegation.
Porter said he had the backing of Morrison.
Australia's Minister for Industrial Relations Michaelia Cash will stand in as acting Attorney-General while Porter takes leave, a spokesperson for Morrison said.
The government has been under pressure over its handling of unrelated allegations by three former staff members of the ruling Liberal party that they had been raped by an unidentified colleague.
One victim who went public with her allegations has made an official police complaint.