Cambodia will receive $35 million in extra development assistance over the next four years in return for resettling refugees processed in Australian detention centres on the Pacific island of Nauru, Australian officials have revealed.

Immigration Minister Scott Morrison divulged details of the secretive arrangement to the Australian Broadcasting Corporation this morning before departing to Phnom Penh to ink the deal with Interior Minister Sar Kheng.

According to Morrison, resettlement "should begin later in the year".

In return, the Australian government would give $35.2 million (AUD$40 million) over the next four years to "support various overseas development aid projects", in addition to the current $69.5 million aid budget for Cambodia.

"Now this is on rice-milling projects, on land-mine-clearance projects and things of that nature, and electoral-reform issues. So these are good projects. They're projects that are very worthy," Morrison told the ABC.

Morrison has also said there will be "no cap" on the number of refugees allowed to resettle in Cambodia, though settlement would be "strictly voluntary".

This means that more than 1,000 asylum seekers currently detained on Nauru could eventually make their way to Cambodia if they are found to be refugees and if they choose to be resettled here under the scheme.

More than 200 people on Nauru have already been found to be refugees.

"Support will be tailored to the needs of [refugees] as part of a package of measures that will go to their resettlement, which is designed to make them self-reliant as quickly as possible," Morrison was quoted by the Associated Press as saying on Thursday.

This morning he confirmed that resettlement packages would be funded by Australia.

Cambodia's Ministry of Foreign Affairs issued a statement on Wednesday ending months of speculation over the signing of the controversial deal, which has been shrouded in secrecy since it was first broached by Australian Foreign Minister Julie Bishop in a meeting with Prime Minister Hun Sen in February.

Minister of Interior Sar Kheng told reporters in Phnom Penh yesterday that the plan will initially involve a small number of refugees moving to Cambodia under a pilot phase of the project.

"We will try to take four or five people, maybe two or three [initially]," he said. "We will keep working to ensure [the deal is in our] interest."

He added that the exact numbers of refugees who might be resettled in Cambodia would be disclosed today after the signing.

The announcement that an agreement had been reached was met by condemnation from Australia's opposition and human rights groups, who have denounced the proposal since its inception was made public.

Australia's shadow minister for immigration, Richard Marles, said in a statement yesterday that the Abbott administration should reveal the details of the plan.

"Scott Morrison needs to provide immediate information about what deal he is about to do with Cambodia," he said. "Morrison must also explain why he believes Cambodia is an acceptable location for resettlement when he so fervently denounced Labor's Malaysia Arrangement," he added, referring to a 2011 deal to swap refugees with Malaysia that Morrison's liberal party opposed at the time. It was later struck down in an Australian court.

Also yesterday, Morrison tabled a bill in the Australian parliament that seeks to introduce a new temporary visa system for refugees and remove references to the Refugee Convention from the country's migration laws.

However, legal experts yesterday said the bill, if it passes, would not affect the refugees at Australia's offshore detention centre on Nauru, who would be offered resettlement in Cambodia.

"The Australian government is trying to find a way through the current unsustainable situation of thousands in limbo both in Nauru, [Papua New Guinea] and on the Australian mainland," Joyce Chia, senior research associate at the Kaldor Centre for International Refugee Law, told the Post. "[Temporary visas] are intended to provide a way out of the unsustainable problem of not processing asylum seekers within Australia, while still maintaining their commitment never to give these people a pathway to permanent residency."

Australian Greens' Senator Sarah Hanson-Young said most refugees would not accept the new visas.

"Scrapping the Refugee Convention from Australian law is an appalling move that clearly shows what this government thinks when it comes to protecting those in need," she said. "Under these new rules, the road to permanent residency is long, extremely narrow and most refugees will never make it."

UNICEF Australia, Save the Children, Plan International Australia, World Vision, Amnesty International, the Refugee Council of Australia, International Detention Coalition and Children's Rights International expressed "grave concerns" for the welfare of children who might be resettled in a statement yesterday.

"This planned deal is inappropriate, immoral and likely illegal," former Australian Chief Justice Alastair Nicholson, speaking on behalf of the alliance, said. "It is inappropriate because Cambodia has no capacity within its social sector to take an influx of refugees. Immoral because these vulnerable people are Australia's responsibility, and while we await the detail, it appears illegal in contravening Australia's humanitarian and refugee obligations to vulnerable children and families."

On August 31, there were 1,233 asylum seekers in Australian immigration detention on Nauru, including 222 children.

But according to Human Rights Watch, as of September 18, only 250 refugee status determinations had been carried out, with 206 people recognised as refugees.

A small, unknown number of the 206 recognised refugees are already living in communities in Nauru on temporary visas and would be some of the first eligible for resettlement here.

During a discussion of human rights in Cambodia at the United Nations Human Rights Council in Geneva on Wednesday, Australia sought to assuage concerns about the scheme.

In a short statement, an Australian delegate told the council that "Australia would use its expertise, experience and funding to help Cambodia to strengthen settlement support provided to refugees".

Although Australia had in January criticised Cambodia at the rights council for abuses including "disproportionate violence" against protesters and detention without trial, it steered away from such topics yesterday.