Closing down offshore immigration detention centres and moving asylum seekers to the mainland for processing in the community would save the federal government $2.9bn.

The Parliamentary Budget Office (PBO) has crunched the numbers on an Australian Greens election policy to shut down the centres on Nauru and Papua New Guinea’s Manus Island.

Both centres have faced international criticism for squalid conditions, deaths and alleged human rights abuses.

The Greens propose a 30-day limit on asylum seekers being held in detention on the mainland before people are moved into community detention.

The PBO calculated that $2.9bn would be saved over four years from 2015-16 if offshore asylum seekers were sent to the mainland.

However, changes to the arrival numbers of asylum seekers by boat would significantly alter the financial impact, the office said.

“This costing is considered to be of low reliability due to the considerable uncertainty around the magnitude of the potential behavioural responses particularly the number of boat arrival asylum seekers,” it warned in its costings.

The Greens senator Sarah Hanson-Young said the money saved could be channelled into education and health spending.

“The government is blowing that money on exposing men, women and children to abuse on Nauru and Manus Island,” she said.

“It’s time for us to close the offshore detention camps and start treating people the way we would want to be treated.”

As prime minister, Kevin Rudd closed down the Nauru and Manus Island centres in 2008 as part of an election promise.

In 2011 and 2012, Labor reversed the policy and reopened both centres under the then prime minister Julia Gillard in response to a dramatic increase in asylum seeker boat arrivals and drowning incidents.

The federal government has been working to clear a backlog of 30,000 onshore asylum seeker claims.