A report by UNICEF and Save the Children found operating the detention system on Nauru and Manus Island had cost $3.6 billion since the reintroduction of offshore processing in 2012.
The report said shutting offshore detention facilities would save $2 billion over four years.
Turnbacks are estimated to have cost nearly $300 million since Operation Sovereign Borders began.
The government has also spent $40 million on the failed Cambodia resettlement scheme and $275 million on "regional cooperation activities", which the government gives to countries in the region to “encourage” them to deter asylum seekers from attempting to get to Australia.
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