SHUTTING down offshore immigration detention centres and moving asylum seekers to the mainland for processing would save the Federal Government almost $3 billion, Parliamentary Budget Office data show.
The PBO calculated that closing offshore detention facilities would save $2.47 billion in 2015-16 budget forward estimates.
During the same period, transferring offshore detainees to onshore detention, with a 30-day time limit, was estimated to save a further $448 million.
However, the office warned the figures had low reliability due to unpredictable factors such as the number of asylum seekers arriving by boat, which would alter the financial impact.
The Australian Greens proposed shutting down the centres on Nauru and Manus Island and imposing a 30-day limit on asylum seekers held in detention on the mainland before they were moved to community detention.
Greens immigration spokeswoman Senator Sarah Hanson-Young said the government was spending billions of dollars on offshore immigration that could be put towards improving health and education.
"The conditions in these centres are appalling and the government is forcing thousands of people to live in squalor and fear," she said.
"The Greens want to see the men, women and children who are in need of protection brought back to Australia so that they can live in safety."