The country's strongman Prime Minister Hun Sen approved an Iranian couple, Iranian man and a Rohingya Muslim from Myanmar living permanently in Cambodia, one of Asia's poorest nations.

General Khieu Sopheak​, spokesman for Cambodia's foreign minister, told the Phnom Penh Post that when the refugees arrive in the capital Phnom Penh remains undecided.

Australia earlier this month pre-empted the decision and flew the refugees secretly from the tiny Pacific island of Nauru to Darwin where they have been staying in immigration department accommodation.

Under the agreement signed at a champagne-sipping ceremony in Phnom Penh last year Cambodia has the right to veto any refugee who agreed to give up their hope of living in Australia to resettle in Cambodia.

The Iranian couple and two single men were the only refugees among hundreds on Nauru who agreed to accept about $15,000 accommodation for a year, training, living costs and other benefits costing tens of thousands of dollars to make the journey.

Australia's immigration minister Peter Dutton has said he anticipates other refugees on Nauru will change their minds after the first group arrives in Cambodia.

But if no other refugees agree to go Australian taxpayers will be forking out more than $10 million for each of the four who are now expected to be flown to Phnom Penh within days.

A Cambodian delegation visited Nauru in April to assess the refugees' applications.

Mr Dutton has complained in the past that refugees on Nauru had been pressuring others on the island not to accept the deal.

Cambodian officials were "shocked" to learn last month that Australian immigration officials on Nauru had told refugees and asylum seekers they could fly to Cambodia as early as April 20.

Refugee advocates say among the promises made to asylum seekers was to have their applications for refugee status fast-tracked if they agreed to go to Cambodia.

The Abbott government announced in this month's budget that Australia's aid to Cambodia would be left intact, despite slashing aid to other south-east Asian nations.

After the refugees arrive in Phnom Penh they will be assigned a case manager and be provided villa or hotel accommodation.

They will be assisted by the International Organisation for Migration, a multi-nation organisation set-up to assist refugees, asylum seekers and migrants.

The United Nations refugee agency has refused to be involved in the agreement which has been criticised by Cambodian opposition parties, many non-government organisations in Phnom Penh and human rights and refugee advocacy groups.

Cambodia has a poor human rights record and offers little support to millions of its own impoverished people.

Almost all of the more than 60 refugees already living in the country say they have faced discrimination, been denied basic rights and are desperate to leave.