He will leave his Canberra office on May 12, two days before his 70th birthday.

Respected former Communications Department chief Drew Clarke has also drawn the curtain on a distinguished public service career with his resignation as Chief of Staff to Prime Minister Malcolm Turnbull.

Mr Richardson has led the mammoth department since 2012. His tenure at Defence was part of a 48-year contribution to public service and national security, Mr Turnbull said on Wednesday morning.

Before he took the helm at Defence, Mr Richardson was secretary of the Department of Foreign Affairs and Trade from 2010 to 2012, ambassador to the United States from 2005 to 2010 and director-general of ASIO from 1996 to 2005.

He also worked in Immigration and Prime Minister and Cabinet, and was principal adviser to prime minister Bob Hawke in the early 1990s.

The 69-year-old broke the news to his 17,000 staff in an internal announcement on Wednesday morning.

"Brendan Sargeant, associate secretary, will be acting secretary pending the appointment of my successor," he wrote.

"I thank everyone for their professionalism, commitment and achievements since October 2012.

"I have thoroughly enjoyed my 48 years in the public service.

"I am delighted to have had the opportunity to work in the Department of Defence and, in particular, to have met many of you personally around the country and overseas."

Malcolm Turnbull paid tribute to the career public servant on Wednesday.

"In this near half century of service, Mr Richardson has served 12 prime ministers with consistent professionalism," the Prime Minister said.

"His commitment to public service, his strategic insight, his candour and integrity have made him a trusted adviser to governments from both sides of Australian politics.

"On behalf of the government and the Australian people, I thank Mr Richardson for his leadership and service and I wish him and his wife, Betty, all the very best for the future."

The praise for Mr Richardson came from across the political divide with Labor Defence spokesman Richard Marles and his portfolio colleagues in paying tribute to the retiring mandarin.

"Dennis' advice and insight has helped governments of both stripes make decisions which have shaped our country's place in the world," Mr Marles said.

"Dennis' personal integrity and judgement meant his advice was always given heavy weight in Labor's national security considerations, whether in government or out."

The commercial sector also expressed its respect for the veteran public servant with the Australian Industry Group publishing a statement praising Mr Richardson's contribution.

"The decision by Dennis Richardson to resign from Defence will see the departure of one of Australia's most dedicated, long serving departmental heads," AIG chief executive Innes Willox said.

Mr Clarke worked as Secretary of the Department of Industry and Resources from 2010 until 2013 when he was appointed as secretary of the Communications Department.

Mr Turnbull picked Mr Clarke as his chief of staff soon after seizing the prime minister-ship in 2015 and the public servant was credited with restoring a more traditional cabinet-process style to the Coalition government.

He will be succeeded by former ambassador to Iran and Defence adviser Greg Moriarty.