Former defence minister David Johnston is being tipped to fill the role of Australia's consul-general to Hong Kong and Macau.

He is the latest in a growing line of Coalition ministers and members to leave Parliament ahead of the election, several of whom have government appointments to ease them into retirement.

Senator Johnston was the only ministerial casualty of the Abbott Government, who stumbled by saying "I wouldn't trust [the Australian Submarine Corporation] to build a canoe" and fell when Tony Abbott reshuffled his Cabinet in December 2014.

His departure has opened up a Senate spot in Western Australia.

Senator Johnston will join Joe Hockey in the diplomatic ranks. The former treasurer is now Australia's ambassador to the United States.

Another job has also been created for departing MP Philip Ruddock, who is now the first envoy on human rights.