"Linda has the talent,the experience,the knowledge and the determination to get the job done,"Mr Morrison said.
Government MPs believe Mr Ciobo has been angling for an overseas posting and that is behind his swift departure from cabinet.
Mr Morrison said keeping Mr Pyne in place despite his retirement was the"responsible course of action"to ensure continuity in an important strategic area.
Already reeling from the loss of Mr Turnbull in August,Mr Pyne's retirement is a fresh blow to the Liberal Party's moderate wing. Other prominent members preparing to leave Parliament include former foreign minister Julie Bishop,Jobs and Industrial Relations Minister Kelly O'Dwyer and Mr Laundy.
The Prime Minister has played down the impact of retirements,noting eight Labor MPs will not recontest the poll.
But Mr Shorten told The Sydney Morning Herald andThe Agethe government was"a farce".
"The Liberals and Nationals are looking after themselves and ignoring the Australian people,"he said.
"They are divided,unstable and more worried about themselves,their jobs and their donors than anything else."
Finance Minister Mathias Cormann is the only minister still in the same cabinet position after the Coalition's five years in office.
Mr Pyne,who has been in Parliament for 26 years,is one of the Liberal Party's leading moderate figures but has told supporters over recent days that he has been deeply unhappy in Canberra in the six months since Mr Turnbull's demise.
His mentor and friend Amanda Vanstone said the Defence Minister deserved to go out on his own terms.
"There are people who’ve stayed for 30 years in Parliament and they are figures of derision,"she said.
"If he has faced the reality of being a figure of derision by trying to hang around for 30 years and decided it’s time to get out,good luck to him. Look the tiger in the eye and deal with it. That’s Christopher."
Mr Pyne holds his Adelaide electorate of Sturt on a margin of 5.4 per cent,but his departure has given Labor a chance of snatching the seat.
Liberals believe the chief-of-staff to South Australian Premier Steven Marshall,James Steven,is the heir-apparent to Mr Pyne,but some local members want a woman and have floated Courtney Morcombe,Mr Marshall's policy director and the wife of federal Trade Minister Simon Birmingham.
Mr Ciobo,who supported Home Affairs Minister Peter Dutton's botched bid to become prime minister,holds his Gold Coast seat of Moncrieff on a 14.6 per cent margin.
His departure gives Mr Dutton a chance to switch to a safer seat,but this option is considered unlikely.
Mr Pyne and Mr Ciobo are entitled to parliamentary pensions because they were elected before the scheme closed to new entrants in 2004. Preliminary calculation shows Mr Pyne would be entitled to an annual pension of about $220,500 a year and Mr Ciobo about $188,000.
Jobs and Industrial Relations Minister Kelly O'Dwyer,Human Services Minister Michael Keenan and Indigenous Affairs Minister Nigel Scullion have all announced their departures from politics at the election. The ministers will stay in their portfolios until the election is called.
Ms Reynolds said her background had prepared her for the new cabinet posting.
"I understand the challenges we confront regionally and globally. I know what it takes to ensure the men and women who serve our nation in the ADF have the best possible equipment – and wherever possible it is made here in Australia."