The first group of refugees transferred from Nauru to Cambodia under a controversial deal have all left the country — less than a year after arriving.

The group of two Iranian men, an Iranian woman and a man from Myanmar arrived in Phnom Penh on June 4 last year.

The ABC understands the last of the original group — an Iranian man — left in recent weeks.

Cambodia agreed to take refugees from Nauru who tried to reach Australia by boat in exchange for $40 million aid from Australia.

The Federal Government also offered to pay the International Organisation for Migration (IOM) $15.5 million to support those who moved there.

Of the five refugees who have been transferred from Nauru overall, just one Rohingya remains in the South-East Asian country.

Cambodian officials will reportedly fly to Nauru next month to interview two more refugees who have agreed to resettle.

An Iranian husband and wife returned to their country of origin in March, just five months after an ethnic Rohingya man returned to Myanmar.

The Cambodian Government reportedly said the man asked to go because he was homesick.

These earlier departures prompted strong criticism from the Opposition, whose immigration spokesperson Richard Marles slammed the resettlement deal as "an expensive joke".