Former deputy prime minister Tim Fischer has criticised the Trump administration's failure to appoint a new US ambassador to Australia, arguing the delay is now damaging the long-standing alliance.

Key points: Tim Fischer says failure to appoint new US ambassador is damaging the relationship between Australia and the US

Tim Fischer says failure to appoint new US ambassador is damaging the relationship between Australia and the US The Trump admin has left several high-profile ambassador posts vacant

The Trump admin has left several high-profile ambassador posts vacant America's top military commander in the Pacific has reportedly been offered the job

The United States has been without an ambassador in Australia since September 2016 and Mr Fischer said the long wait was now "beyond acceptable" and bordering on a "diplomatic insult".

While there is bipartisan praise for James Caruso — who is acting in the role as Charge D'Affaires in the absence of a full-time ambassador — there are concerns the vacancy is adding to appearances of a strained diplomatic relationship.

"We had a saga of the refugee deal earlier last year and now the fallout from the correct diplomatic activity of Alexander Downer, so we're obviously not at the bottom of the list but we're near the bottom of the list," Mr Fischer told AM.

The PM met Donald Trump in NYC last year, but there is still no US ambassador in Canberra ( Reuters: Jonathan Ernst )

Prime Minister Malcolm Turnbull had been quick to dismiss concerns on Monday, saying key diplomatic postings were often delayed when a new US Government was elected.

"The relationship is in excellent shape and the connections between Australia and the United States are so diverse, numerous, so strong, and indeed, our relationship through the State Department is excellent as well," Mr Turnbull said.

"So, it's not a matter of concern."

The Trump administration has left several high profile ambassadorial posts vacant for a long time.

For example, South Korea has gone without an ambassador for several months despite the high-level military and diplomatic cooperation with the United States.

The ABC understands Mr Turnbull will visit Washington with Australian business leaders in coming months.

Malcolm Turnbull says the relationship between the US and Australia is strong. ( ABC News: Marco Catalano )

Top admiral reportedly offered the job

Acting Opposition leader Tanya Plibersek disagreed with the Prime Minister on Wednesday, saying she was concerned it appeared a diplomatic relationship with Australia was not a high priority for the Trump administration.

Mr Fischer conceded the diplomatic vacancy did not reflect the strength of Australia's broader relationship with the US, evidenced by last year's visit of Vice President Mike Pence.

Admiral Harry Harris was tipped to take on the role of US ambassador to Australia. ( Reuters: Yuri Gripas )

He said it was unlikely an ambassador would be announced in the next three months.

America's top military commander in the Pacific, Harry Harris, was tipped for the diplomatic posting in August and told an Australian audience a month later that it would be "a good job".

The Washington Post reported late last year that Admiral Harris, who met with Prime Minister Malcolm Turnbull in New York in May, had been offered the job.

Admiral Harris' military post is expected to end sometime in 2018.