The federal government has spent more than $1 billion this financial year to house about 2200 asylum seekers in offshore detention centres in Papua New Guinea and Nauru.

Running the detention centre on Manus Island has cost taxpayers $632.3 million, and the operational cost of Nauru was $582.4 million, a Senate estimates hearing was told on Monday.

$632 million a year: The detention centre on Manus Island. Credit:Alex Ellinghausen

Deputy secretary of immigration Mark Cormack said the costs – for 1167 asylum seekers on Nauru and 1060 on Manus Island – included health expenses, escorting asylum seekers to and from the centres, staff costs and payments made to both countries to support the detention facilities.

A spokeswoman for Immigration Minister Scott Morrison defended the costs, saying the running of the two centres underpinned the government's border protection regime and the costs had declined by "almost $100 million" compared with the previous financial year.