Former prime minister Kevin Rudd has slammed the United States' decision to withdraw its nominated ambassador to Australia.

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 ahead of Donald Trump's planned meeting with North Korean leader Kim Jong-un.

"(It) basically says Australia, from President Trump's perspective, is a second-class ally," Mr Rudd told 7.30.

Danger Trump 'taking Australia for granted'

Donald Trump is expected to meet Kim Jong-un some time this year. ( Reuters, Kevin Lamarque )

Mr Rudd said he understood the reasoning behind the decision, given the strategic importance of the Trump-Kim meeting, but said he believed it was indicative of the view of Australia within the Trump administration.

"All Australians expect any administration in the United States to take our alliance seriously," he said.

"So to chop and change at the last minute like this and take a good candidate, Admiral Harry Harris, and take him from our grasp and send him to Seoul is, I think, a bad signal to the wider public community in Australia about the importance which this Trump administration attaches to Australia."

"There is a danger that the Trump administration begins to take Australia for granted."

Trump has 'not treated Australia particularly well'

Malcolm Turnbull with Admiral Harry Harris during his visit to Pearl Harbour in 2016. ( AAP: Supplied )

The position of US ambassador to Australia has been vacant for more than 18 months, since John Berry left in 2016.

Publicly there has been no expression of concern from Canberra about the delay in appointing a new ambassador.

But Mr Rudd said the vacancy in such a strategically important position had ramifications for Australia.

"The virtue of ambassadors in both capitals is that they can quickly, immediately and authoritatively get through to the head of each government or administration," he said.

"If I was in Prime Minister Turnbull's office right at the moment, I think it would be full of colourful and rich language.

"The President of the United States has, so far, not treated Australia particularly well."

Harris appointment 'will not be welcomed by the Chinese'

Admiral Harry Harris will now be an envoy to South Korea. ( Reuters: Yuri Gripas )

While Mr Rudd acknowledged Admiral Harris's abilities, he said the appointment would not be welcomed by all parties with an interest in the Korean peninsula.

"I think I can say fairly safely that the appointment of Admiral Harris to Seoul will not be welcomed by the Chinese, because the Chinese have dealt with Admiral Harris in PACOM (US Pacific Command) and have regarded him as upfront and in your face," he said.

"That's not necessarily a bad thing in itself, I'm simply describing the reality."

Earlier today Foreign Minister Julie Bishop played down the news.

"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.