The federal coalition's plan to erect a tent city for asylum seekers on Nauru could cost close to $2 billion over four years, the Parliamentary Budget Office (PBO) says.
The Australian Greens asked the PBO to provide costings for the upgrade to Australia's processing centre, which is a signature coalition policy to address boat arrival numbers.
The PBO said the plan could cost $1.9 billion over four years and that it would have an "ongoing impact" beyond the forward estimates.
Greens Senator Sarah Hanson-Young said the figures showed that offshore processing was massively expensive and inhumane.
"The coalition are doing everything they can to hide the price of their policies, but the Australian people deserve to know the true cost of the cruelty that their tax dollar is funding," she said in statement on Tuesday.
Labor also plans to send more people arriving on boats to Nauru for processing, although it hasn't specified how many extra beds it will add to the facility.
Opposition Leader Tony Abbott says a coalition government will immediately expand capacity on Nauru to 2000 and will add another 3000 beds in five years.
Meanwhile, Australian authorities have transferred another 38 asylum seekers, including 14 children, to Nauru over the past few days, taking the total number of detainees on the island to 543, the Nauruan government says.
Another 41 single adult males were also transferred to Papua New Guinea's Manus Island on Tuesday, the immigration department says.