Immigration Minister Peter Dutton has conceded fewer than 20 of the 850 asylum seekers held on Manus Island have voluntarily resettled in Papua New Guinea, as the government struggles to deal with the closure of the controversial detention centre.

Speaking a day after the PNG government confirmed it would end detention on Manus following an April court ruling finding the centre was in breach on the country's constitution, Mr Dutton lashed out at media reports about conditions in offshore processing and said detainees held by Australia received appropriate mental health care.

Asked if it was true that some men who had resettled voluntarily in PNG had later sought to return to the detention centre, Mr Dutton said refugee advocates in Australia were to blame for sending social media messages to detainees telling them not to agree to resettlement offers.

Speaking on ABC radio on Thursday, Mr Dutton said Australia was supporting hundreds of asylum seekers and refugees who had returned to their countries of origin.