One refugee left in Cambodia under deal

There is only one refugee left standing in Cambodia under the federal government's $55 million refugee resettlement deal.

An Iranian couple voluntarily left the country in late February, the Khmer immigration department spokesman Tan Sovichea told the Cambodian Daily newspaper.

A fourth refugee, an ethnic Rohingya, who agreed to move to Cambodia from Nauru, went back to Myanmar (Burma) last October.

Opposition immigration spokesman Richard Marles said the couple's departure shows it's "all but over for the botched deal".

"The inability of this government to secure a meaningful resettlement arrangement with a credible third country is a serious failure on the part of Malcolm Turnbull," he said.

Cambodia has indicated it's not prepared to accept any more refugees.

A spokeswoman for Immigration Minister Peter Dutton said refugees could elect to return to their country of origin at any time.

The government remained committed to supporting the Cambodian government to implement settlement arrangements.

It was still encouraging refugees on Nauru to explore Cambodia as a resettlement option.

Australian Greens senator Sarah Hanson-Young said the deal had been a blatant waste of taxpayer dollars.