The cost of Australia's immigration detention system has blown out by more than $1 billion as the Turnbull government forks out for charter flights, accommodating asylum seekers and funding the governments of Nauru and Papua New Guinea.

The mid-year economic and fiscal outlook (MYEFO) shows the government will spend an additional $499.7 million over two years from 2016‑17 to manage so-called "illegal maritime arrivals" who are in Australia while their immigration status is resolved.

The money will help manage the "legacy caseload" – about 30,000 asylum seekers mostly living in the community. They arrived before July 2013, when former prime minister Kevin Rudd announced that those who arrived on unauthorised boats would not be resettled in Australia.