Environment Department boss Finn Pratt will retire in October after a public service career spanning more than three decades.
Prime Minister Scott Morrison announced Mr Pratt's decision to depart his role on Monday and will need a replacement at the helm of the 2300-strong agency.
The department leader's retirement on October 11 leaves a second secretary position open following Steven Kennedy's move from the Infrastructure Department to lead Treasury, and Phil Gaetjens' elevation to secretary of Prime Minister and Cabinet.
Mr Pratt's public service career stretched 36 years. He delivered reforms in employment services, industrial relations, the National Disability Insurance Scheme and aged care.
He's led the Department of the Environment and Energy since a major reshuffle of departmental secretaries in September 2017 and has overseen the expansion of its work in the Antarctic.
Mr Pratt led the Department of Human Services and the Department of Families, Housing, Community Services and Indigenous Affairs under the previous Labor government.
He moved to the Social Services Department where he was secretary after the Coalition came to power in 2013.
The long-time bureaucrat received a public service medal in 2008, and in 2015 was appointed an officer of the Order of Australia.
Mr Morrison this month announced he would re-appoint Mike Pezzullo as Home Affairs Department secretary and Chris Moraitis would continue as Attorney-General's Department boss.