A prominent US anti-immigration think tank has warned a refugee resettlement deal won’t last Donald Trump’s first day as president.

“My sense is that when the word gets out on this, it’ll be dead on arrival,” Mark Krikorian, executive director of the influential Centre for Immigration Studies, told Fairfax Media on Monday, predicting a “firestorm” of opposition from anti-immigration activists.

“I don’t expect any Republicans will defend it; I can’t see a lot of Democrats defending it either,” he said.

Mr Krikorian said Australia had 68 days to pull out of the deal before Mr Trump’s inauguration on January 20.

He acknowledged the upside for Australia in the deal, but queried what was in it in for the US.

“I’m not sure why this is a good idea for the US – it’s absurd. I’m not sure why we’d be taking them off your hands.”

Turnbull can’t guarantee deal or provide numbers

Prime Minister Malcolm Turnbull is heralding the new deal, but can’t say how many will be sent or if it will even survive past January.

However, he has revealed the amount to go from Manus Island and Nauru will have to fit within America’s existing annual intake.

The prime minister insists the deal was struck with the Obama administration some time ago and the issue hasn’t been raised with President-elect Trump.

“We deal with one administration at a time. You don’t discuss confidential matters with one administration with a future administration,” he told the Nine Network on Monday.

Mr Turnbull believes Mr Trump — who campaigned to ban Muslim migration — will pursue the national interests of the US once he is sworn in on January 20.

But he says there is a common national interest in defeating people smugglers and defending the integrity of the seas.

“Our two nations, Australia and the United States have had a long co-operation on matters of this kind which have enabled us to pursue and support our mutual and respective humanitarian and indeed national security objectives,” he told the Seven Network.

Repeatedly asked how many will be sent under the arrangement, the prime minister said no number had been set but it will be a substantial amount.

He insists it is a one-off and will not apply to any future arrivals.

The timing will be up to the United States who will accept referrals from the United Nation’s refugee body, with officials to arrive this week.

“The process will continue for some months,” he said.

“The United States won’t be short cutting their security or health checks.”

The UNHCR wants the government to find a solution for all asylum seekers in offshore detention.

“It’s important that they don’t remain in Nauru and Papua New Guinea, where they’ve stayed for far too long and indeed languished in limbo,” external relations officer Catherine Stubberfield told ABC radio.