Human rights and refugee advocacy groups say Cambodia's treatment of the Montagnards, including children, as "illegal migrants" violates its obligations under the 1951 Refugee Convention and other international laws.

Australia has used Cambodia's signatory to the convention to justify sending refugees from its detention centre in Nauru to the impoverished nation.

Scott Busby, a high-ranking US official visiting Phnom Penh, urged Cambodia to register and allow the Montagnards to have their claims for refugee status heard by the United Nations refugee agency.

But Cambodia's foreign minister, Hor Namhong, rejected the call, saying if the Montagnards were allowed to register, up to a million Vietnamese claiming to be Montagnards could enter Cambodia.

Montagnards are people from Vietnam's central highlands who claim a litany of grievances from Vietnam's communist government, including confiscation of ancestral homelands, loss of agricultural land to settlers from low-land Vietnam, lack of freedom of worship and the denial of other basic rights.

Vietnamese authorities deem the Montagnards' adherence to Protestant Christianity as the "wrong religion" and brutally crushed their protests last year.

Many Montagnards fought with the US side during the Vietnam War.

Cambodian police have deported at least 54 Montagnards from its Rattanakiri province in recent months.

While Cambodia's strongman prime minister Hun Sen accepted $40 million in additional aid from the Abbott government to take refugees from Nauru, critics say his country's track record of protecting the rights of refugees and asylum seekers is poor.

In 2009, Cambodia prompted international condemnation when it deported 20 ethnic Uighur asylum seekers to China days after Beijing announced a huge increase in aid funds to the country.

Interior ministry spokesman Khieu Sopheak said at the time Cambodia was a neutral country and that "taking refugees from any country … is against Cambodian constitutional law."

Australia has been widely criticised in Cambodia and internationally for sending refugees to the country, one of the world's poorest nations where Mr Hun Sen has for more than three decades brutally crushed dissent and political opposition.

Australia has imposed strict secrecy around the arrival of the Nauru group of four refugees after Cambodian authorities approved them becoming permanent residents two weeks ago.

The four - an Iranian couple, an Iranian man and Rohingya Muslim man from Myanmar have accepted $15,000, villa accommodation, income support, health insurance, training and other benefits to agree to give up their hopes of living in Australia to take a one-way ticket to Cambodia.

But the benefits will dry up in 12 months, leaving them to fend for themselves in a country where millions of people live in dire poverty and where education and health services are poor.

Hundreds of other refugees on Nauru refused Australia's offer.

As well as the $40 million in additional aid to Cambodia, Australian taxpayers have so far forked out $15.5 million to get the four refugees to the country, according to evidence at a Senate committee hearing in Canberra.

Immigration minister Peter Dutton has declined to comment on the refugees.

By Lindsay Murdoch - The Sydney Morning Herald - June 3, 2015