Mr Warner previously served as director-general of ONI’s predecessor, the Office of National Assessments.
He was also the director-general of Australia’s overseas intelligence agency, ASIS, as well as the secretary of the Department of Defence.
One of the favourites speculated to replace Mr Warner is cabinet secretary Andrew Shearer, a national security expert and former deputy director-general of ONI.
Mr Shearer also worked as a national security adviser to Tony Abbott and John Howard, and was a senior advisor at Washington think tank, the Centre for Independent and Strategic Studies.
Other potential candidates include current ONI deputy director-general Paul Taloni, the former director of cyber spy agency the Australian Signals Directorate, Paul Grigson, deputy secretary of the Department of Home Affairs and Michelle Chan, Mr Morrison’s national security advisor.
Senior government sources confirmed there were a number of candidates and no decision had been made.
Mr Morrison paid tribute to Mr Warner, saying he had known the intelligence chief for many years and he had an “extremely long and distinguished career serving Australia’s interests and defending and protecting Australia’s interests”.
“He’s a great Australian, who has done an outstanding job, spanning some four decades in areas of national security, foreign policy and so many other areas,” he said.
“His expertise, his experience, his knowledge of these areas is, I would say, without peer and we have been well-served by him in this time, both in this role as the director-general of the Office of National Intelligence but also as director-general of the Australian Secret Intelligence Service.
“Nick will still be in a position to be supporting us in any number of other arrangements on a less formal basis going forward into the future.”
The ONI was established after the 2017 review into Australia’s intelligence agencies recommended its predecessor needed to be expanded to have a direction and supervision role over the nation’s other spy agencies.