Amnesty International Australia says asylum seekers who were recognised as refugees at the Nauru processing centre months ago are still being detained by Australia.
An amnesty spokesperson, Graeme McGregor, says the reason why they have not been released is uncertain, as is the number of refugees concerned.
He says some have speculated Nauru became concerned about the refugees entering the community after a riot at the centre.
But he says the Australian government has always said asylum seekers will only be detained while they were being processed for refugee status.
He says Nauru never agreed to re-settle the refugees, and Australia's policies do not allow them to be settled there - so they refugees have been left in limbo.
"These people are being detained for no good reason at all, arbitrarily. So we're calling on the government ultimately to end offshore processing, but in the interim to ensure that those people recognised as refugees by the processing in Nauru and in Manus Island in Papua New Guinea are promptly re-settled either in Australia or elsewhere."