Australia has spent $3.5 million out of a $15 million budget allocation so far on operating a failed refugee resettlement deal in Cambodia.

Cambodia agreed in 2014 to resettle refugees from Nauru in exchange for a $40 million aid sweetener.

Only five people have been sent to the impoverished Asian country and only one remains - the others have opted to return to their country of origin.

During a Senate hearing on Friday, immigration department officials tried to justify the expenditure.

"What an earth is that money being spent on?" Labor senator Murray Watt asked.

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Department secretary Michael Pezzullo said the money was going to the contract with the International Organisation for Migration and a small amount to another service provider Connect.

An official said the service providers had to be there in readiness for more refugees, however those on Nauru have not been enticed to move to Cambodia

Khmer language tuition, cultural and vocational training, health services and accommodation was on offer to refugees who resettled under the program.

Asked about the threat of corruption in Cambodia, Mr Pezzullo insisted immigration was monitoring invoices closely and wasn't handing over "bags of cash".

The sole refugee in Cambodia Rohingya man Mohammed Roshid told the ABC in June he was worried about being cut off from support at the end of the year and he complained about poor quality health care.