The Trump administration plans to overturn its nomination of Admiral Harry Harris as the next United States ambassador to Australia, putting him forward as envoy to South Korea instead.

Key points: The post in Australia has been vacant for 18 months

The post in Australia has been vacant for 18 months "Australia is being treated here as a second-class ally," Andrew Shearer says

"Australia is being treated here as a second-class ally," Andrew Shearer says Donald Trump described Admiral Harris as an "outstanding man"

The surprise move comes as President Donald Trump tries to beef up his team in the Korean peninsula, ahead of his proposed historic meeting with North Korean leader Kim Jong-un later this year.

US media reported the decision was triggered by incoming Secretary of State Mike Pompeo. A vote on Mr Pompeo becoming America's top diplomat could happen as early as this week.

Admiral Harris had been due to appear at a confirmation hearing for the Canberra role this morning, but a spokesman for the US Senate Foreign Relations Committee said it had been postponed until May "at the request of the administration".

Foreign Affairs Minister Julie Bishop said acting US Secretary of State John Sullivan told her yesterday that Admiral Harris was being reassigned to another diplomatic post.

"While we would have welcomed Admiral Harris here as ambassador to Australia, we understand that there are significant challenges for the United States on the Korean peninsula," Ms Bishop said.

She said it was not unusual and it had happened before, but said the Government looked forward to a new ambassador being nominated as soon as possible.

"We of course have a very strong and deep relationship with the United States and I am sure they will choose a very appropriate ambassador to take his place in due course."

She said the acting Secretary of State had assured her that announcing a new ambassador to Australia would be a priority.

A US State Department official said the country's relationship with Australia was "steadfast" and that communications were "as good as ever".

The official made assurances the US diplomatic mission in Australia was in good hands while the appointment of a new ambassador was pending.

Admiral Harris (left) has extensive experience and interest in US-Australia relations. ( AAP: Supplied )

An empty chair for more than a year

The post in Australia has been officially vacant for more than 18 months.

Former Australian ambassador to the US Kim Beazley described it as "not ideal" when the first 12 months passed without a US ambassador in Canberra.

At the time, Mr Beazley said technically it did not make a difference that an ambassador was not in place because the embassy was still staffed.

But he noted it was meaningful in the sense that Australia did not have the substantial American presence that an ambassador would bring.

Responding to today's news, Mr Beazley said Australia had no choice but to "suck it up" given the emerging situation on the Korean Peninsula.

"The US would want to quite quickly arrive at an alternative ambassador, but it'll hard to beat the Admiral," he said.

But he played down suggestions the move is an insult to Australia, instead blaming the Trump administration's "inept" and "foolish" handling of its original nominee for the South Korean post, Victor Cha.

"He is a genius — there is nobody outside of Korea who knows more about Korea than him," Mr Beazley said.

"Right now, he sure as heck would be useful."

Labor's defence spokesman Richard Marles said the sooner the Trump administration appointed an ambassador, the better.

But he was reluctant to say Australia was being snubbed by the United States, and pointed out the current Charges d'Affaires was doing a "fantastic job" holding the fort in the meantime.

"Sure, there hasn't been a formal ambassador — but I'd hesitate to say it's vacant," Mr Marles told Sky News.

"We're close friends, I don't think anyone is taking offence."

Australia 'collateral casualty' in Trump's decision

Andrew Shearer, a former national security adviser to John Howard and Tony Abbott, told the ABC it would be "hard to escape a bit of a sense that Australia is being treated here as a second-class ally".

"I think that is regrettable," he said.

"Australia really is, if you like, a collateral casualty here to the shambolic personnel practices of the Trump administration."

Mr Shearer, who now is senior adviser on Asia Pacific Security at the Centre for Strategic and International Studies in Washington, believed there would not be any ramifications from the decision.

"I don't think there is anything sinister to it. I think it is much more typical of the ad hoc moment-to-moment decision making of this particular administration," he said.

"I think the reality is the alliance is so deep and broad, much of our day-to-day business or intel cooperation or defence links or trade and investment goes on uninterrupted."

One insider says the move says more about Donald Trump's White House than it does the US-Australia relationship. ( Facebook: Malcolm Turnbull )

Former deputy prime minister Tim Fischer was less reserved in his interpretation of the move.

"One year is an accident not having a US ambassador, reflecting low priorities for the Australia-US alliance. Nigh on two years will be an insult, and with impact," Mr Fischer said.

'See you in Canberra, Harry!'

Known for his hawkish views on China, Admiral Harris has been commander of the Pacific Command based in Hawaii for the last three years.

He was set to retire later this year before accepting the mission to become US ambassador to Australia.

Admiral Harris has extensive experience and interest in US-Australia relations.

The Trump administration formally nominated Admiral Harris to become the ambassador to Australia in February.

"We're looking forward to sending our newly-nominated ambassador, Admiral Harry Harris, to [Australia] very shortly," Mr Trump said in a press conference with Malcolm Turnbull earlier this year.

"He's an outstanding man. You're going to find that he is a great man."

When the announcement was first made, Mr Turnbull enthusiastically tweeted: "See you in Canberra, Harry!"