Scott Morrison has tapped one of the Coalition's most trusted voices, Arthur Sinodinos, to become the next ambassador to the United States as President Donald Trump raises questions over Australia's role in the FBI inquiry into his 2016 election.

Ahead of what could be a testing period for Australia's relationship with its most important diplomatic relationship, Senator Sinodinos - widely tipped to take a cabinet position in the re-elected Morrison government - will head to Washington.

Next ambassador to the US, Arthur Sinodinos. Credit:Janie Barrett

The term of Joe Hockey, the current ambassador and former treasurer in the Abbott government, is to finish at the end of the year. This enabled Mr Morrison to offer the post to Senator Sinodinos who is expected to take over in January 2020, the start of a presidential election year.

Senator Sinodinos said he was pleased to take up the posting.