The United States could resettle no-one from Australia's offshore immigration centres and still argue it has honoured its deal with the Turnbull Government, according to the Department of Foreign Affairs and Trade.

Key points: Agreement doesn't require US to take in any particular number of refugees, DFAT says

Agreement doesn't require US to take in any particular number of refugees, DFAT says Eventual resettlement number won't impact number of Costa Rica refugees Australia takes in

Eventual resettlement number won't impact number of Costa Rica refugees Australia takes in Amnesty International says there's "no plan B" for those who will be left behind on Manus and Nauru

But DFAT maintains that it expects the United States will take in large numbers of refugees from Manus Island and Nauru, as part of the deal brokered in the closing weeks of the Obama administration.

Andrew Goledzinowski, DFAT's Ambassador for People Smuggling and Human Trafficking, told a Senate committee hearing the agreement did not require the US to take in any particular number of refugees.

When asked by Greens senator Nick McKim whether the US could resettle no-one and still claim it had complied with the agreement, Mr Goledzinowski responded "technically, I think that's the case".

But he also said that the US could end up taking more than 1,250 refugees.

"Whether they end up taking more than 1,250 from Manus and Nauru or significantly less is impossible to say at this stage," he said.

"It could well be that the US eventually chooses to take more than 1,250."

Mr Goledzinowski said the eventual resettlement number would not impact on the number of refugees currently in Costa Rica that Australia takes in, despite Immigration Minister Peter Dutton publicly linking the two deals last month.

Mr Dutton told Sky News that "we wouldn't take anyone until we had assurances that people were going to go off Nauru and Manus".

Graham Thom, Refugee Coordinator for Amnesty International Australia, said there appears to be no plan for those who will be left behind.

Dr Thom told the Senate committee there were concerns not only over the uncertainty surrounding the deal, but also the shortfall in resettlement numbers.

"You're left with 700 people who have suffered for years, stuck on these islands with no plan B," he said.

"This is a problem. Even those who are going through this process, it's going to take months and months, which is going to damage them further."

The latest immigration detention statistics supplied by the Department of Immigration and Border Protection detailed there were 1,241 people in regional processing centres — 861 in Manus Island and 380 on Nauru.

These figures do not include people no longer residing in the centres.

Dr Thom said more than 1,600 people had been recognised as refugees on Manus Island and Nauru, while many remain unprocessed both on the islands and in Australia for medical treatment.

He said there were also concerns over the vetting process for resettlement, saying the latest travel ban signed by US President Donald Trump could lead to additional delays.

"We know that the US system can take some time," he said.

"These people simply do not have that time."

Mr Trump criticised the resettlement deal last month, describing the agreement as "dumb".

He also reportedly accused Prime Minister Malcolm Turnbull of seeking to export the "next Boston bombers" to the US, and complained that the deal was going to kill him politically.

The Department of Immigration and Border Protection also addressed the Senate committee, but took questions on notice regarding the fate of refugees not resettled by the US.

DFAT officials also confirmed that the offer from the New Zealand Government to resettle refugees remained "live".