Prime Minister Malcolm Turnbull has expressed his disappointment Harry Harris will no longer be coming to Canberra to serve as the United States ambassador.

Key points: Harry Harris is being posted to South Korea instead of Australia

Harry Harris is being posted to South Korea instead of Australia Malcolm Turnbull says he has known about the cancellation for "some time"

Malcolm Turnbull says he has known about the cancellation for "some time" He has not spoken with US President Donald Trump since the change

It was confirmed on Wednesday that Admiral Harris would instead be posted to South Korea as an envoy.

Mr Turnbull, speaking from Villers-Bretonneux after Anzac Day services, said he had known about the decision for "some time".

"I am disappointed that Harry is not coming because he is a really good friend, and I think Harry will be disappointed he is not coming to Canberra too because he really loves Australia," he said.

But he said he understood why the Trump administration had decided to send the Admiral to South Korea.

"He is a guy of enormous experience and ability and given the situation on the Korean Peninsula, given the tensions there, I can well understand why the President has decided that the Admiral's expertise and experience is going to be able to be put to better use in Korea than in Australia," Mr Turnbull said.

He would not be drawn on suggestions by former prime minister Kevin Rudd that the decision made Australia look like a "second-class ally".

Australian, US relationship 'deep and intense'

The Prime Minister made assurances the absence of an ambassador was not worrying for Australia's relationship with the United States.

"The relationship between Australia and the United States is so deep and so intense, it operates at so many levels," Mr Turnbull said.

He stressed the current Charge d'Affaires Jim Caruso was doing a good job.

The Prime Minister said he had not spoken with Mr Trump since the overturn, but expected the US would move on with another announcement.

Foreign Minister Julie Bishop 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 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.

Tensions buzz following announcement

The last-minute cancellation of the military commander's nomination to become the US ambassador sparked mixed reactions.

The position has not been filled for 18 months after John Berry wrapped up his tenure in 2016.

Former national security adviser Andrew Shearer said it was "regrettable", but he believed there would not be any ramifications from the decision.

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

Sorry, this video has expired Former national security adviser Andrew Shearer speaks to The World

Former Australian ambassador to the US Kim Beazley said Australia had no choice but to "suck it up" given the emerging situation on the Korean Peninsula.

Mr Beazley and former defence minister Brendan Nelson agreed the appointment had taken too long.

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.

These concerns were echoed by Mr Rudd, who told the 7.30 the change was "dangerous".