Former deputy prime minister Tim Fischer has accused the US of perpetrating a "diplomatic insult" to Australia by failing to appoint an ambassador in Canberra for nearly 18 months, underlining a tension he believes will worsen after this week's revelations about Alexander Downer.

The US has not replaced its former ambassador to Australia, John Berry, since he left the post in September 2016. It is one of many senior international roles the Trump Administration has failed to fill, but the vacancy is considered unbefitting of a close friend and ally.

"This is now bordering on a diplomatic insult," Mr Fischer told Fairfax Media on Tuesday. "We've been downgraded, despite all the nice noises. We are a low priority."

Mr Fischer, who served as Australian ambassador to the Holy See for four years, was a trade minister in the Howard government and deals regularly with diplomats in his role as vice-chair of the international Crop Trust.