The announcement, which prematurely ends the term of former South Australian Labor premier Mike Rann, was made by Foreign Affairs Minister Julie Bishop on Monday.

However, it's not all bad news for Mr Rann. He has been named the new ambassador to Italy, a move foreshadowed by Fairfax Media last month. He will take up the posting in Rome next month.

Mr Downer was Australia's longest-serving foreign affairs minister, from 1996 to 2007.

He follows in his father's footsteps, with Alexander Downer snr holding the high commissioner post from 1963 to 1972.

This is the second time the Abbott government has dumped a former Labor premier from a diplomatic post in favour of a former Howard government minister. Former Victorian premier Steve Bracks was blocked on the eve of his departure for the consul-general post in New York after Mr Abbott came to power last year. Former Howard government minister Nick Minchin was named as his replacement.

Labor's foreign affairs spokeswoman Tanya Plibersek called for an explanation from Ms Bishop why Mr Downer was ''so abruptly installed to the position''.

''It is understood Mr Rann's term was not due to expire for around 18 months,'' Ms Plibersek said. ''In a clear sign of how controversial such a blatantly political appointment is, the Abbott government kept its decision secret until after the … state election in South Australia.

''The previous Labor government inherited Liberal appointments, like Amanda Vanstone as ambassador to Italy, who were given the opportunity to finish their terms. Labor also appointed former Liberal leader Brendan Nelson as ambassador to NATO and Tim Fischer as ambassador to the Holy See.''

Mr Downer was until recently the United Nations' special adviser on Cyprus.