Manus and Nauru detainees leave for US

More than 50 refugees have left Australia's offshore detention centres for a new life in the United States.

Some 25 men being held on Manus Island left on Tuesday from Port Moresby airport in Papua New Guinea.

Another 27 refugees left Nauru on Wednesday, bound for Port Moresby, in the first leg of a 50-hour journey to America.

Among the group were families as well as single men and women from countries including Bangladesh, Iran, Sri Lanka, Somalia and Burma.

They are expected to be dispersed across several states once they land in the US.

Refugee Action Coalition spokesman Ian Rintoul said the refugees were happy to be heading to the US, but uncertainty remained for those who remained in offshore detention.

Mr Rintoul said it had been 10 months since the refugees had their first interviews, and there was no indication of when any others who applied would be accepted into the US.

"There are more than 1700 refugees, including around 150 Rohingyan refugees, on Manus and Nauru who desperately need safety and protection now," he said in a statement on Wednesday.

"The refugees were bluntly told by the US officials that there is no date for more refugees to fly to the US, and that some people will not be accepted."

The refugees arrived in Australian waters by boat years ago and were transferred to the centres under a strict government policy not to allow any such persons to set foot on Australian soil.

They were recently cleared by US authorities for resettlement under a deal struck between the former Obama administration and the Turnbull government.

Up to 1250 refugees are expected to be resettled in the US.