A new report says Australia's asylum seeker detention policy has cost more than $US7.5 billion since it was reactivated in 2013.
The report, published by Save the Children and UNICEF, estimates detention could cost the federal government as much as $US4.3 billion in the next four years.
It analysed government data and expenditure, and estimated the cost of keeping around 2,000 asylum seekers and refugees in Papua New Guinea and Nauru at about $US300,000 per person.
That's compared to about $US24,000 if they were allowed in Australia on bridging visas.
Under the government's policy, asylum seekers who try to reach Australia by boat are held in Nauru and Manus Island on Papua New Guinea and prevented from ever settling in Australia.
The policy has been widely condemned by human rights organisations and the United Nations, but it has bipartisan support from Australia's two main political parties.
The report said the policy is inflicting incalculable harm on asylum seekers, straining bilateral relationships, and damaging Australia's international reputation.
Australia's Parliament has said it will investigate allegations of abuse, self-harm and neglect of asylum seekers detained on Nauru and Papua New Guinea's Manus Island.