The group of 29, including eight children, who flew off the island nation on Sunday is the fifth cohort to depart Nauru under Australia's refugee resettlement deal with the US.

They included two Sri Lankan, one Rohingyan and one Afghan family and single men from Sri Lanka, Afghanistan, and Pakistan, refugee advocates said.

Since resettlements under the deal started in September, 139 refugees have left Nauru and 85 from Papua New Guinea's Manus Island.

However, the Refugee Action Coalition said it was remarkable there were no Iranian, Somali or Sudanese refugees among those who have left this year, despite Australian Border Force officials denying any particular nationalities were banned from resettling in the US.

RAC spokesman Ian Rintoul said US officials were expected to remain in Nauru for another couple of weeks but had not yet scheduled any new vetting interviews for another round of refugees.

Under the deal Australia reached with the previous Obama administration - derided as the worst ever by now-president Donald Trump - the US agreed to take up to 1250 refugees from the offshore detention centres.