The number of asylum seekers and refugees receiving treatment in Australia now outstrips those left on Manus Island and Nauru, following a surge in medical transfers.

Since July, 461 people have been evacuated from Nauru for medical treatment, a 10-fold increase on the previous financial year.

Home Affairs Secretary Mike Pezzullo told a Senate estimates hearing that that brings the total number of people brought to the mainland for medical care or to accompany sick family members to 953, outnumbering the 915 people held offshore.

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Sick refugees and asylum seekers have continued to be sent to mainland Australia, while the reopened Christmas Island detention centre remains empty.

That's despite Prime Minister Scott Morrison saying in February it was necessary to reopen the detention centre at a cost of $185 million to deal with sick people from Manus Island and Nauru who could use new medevac legislation to get to Australia.

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Mr Pezzullo confirmed reports that one person had been transferred under the new legislation which gives doctors more say on medical transfers, but the patient was sent to mainland Australia rather than Christmas Island.

The budget papers revealed the government plans to shut down the centre again by July if it is re-elected in May potentially without a single person being sent there.

More than 500 refugees have now been settled in the United States and 822 people have voluntarily returned to their home countries, Mr Pezzullo told the hearing on Thursday.

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Paladin contract investigation

Taxpayers spend about $1 billion per year maintaining Australia's network of offshore processing centres.

An investigation has been launched into a controversial $423 million contract given to security company Paladin on Manus Island.

Australia's auditor-general is investigating whether the Home Affairs Department "appropriately managed the procurement of garrison support and welfare services for immigration processing centres".

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Mr Pezzullo said he welcomed the audit, noting his department was also undertaking an internal review into the contract.

Paladin provides garrison support and welfare services for Australia's offshore detention regime on Manus Island at a cost of $20 million a month.

The little-known company, whose Australian arm was until recently registered to a beach shack on Kangaroo Island, was awarded the contract in 2017 through a closed tender process.

In February, Paladin's Manus Island staff walked off the job claiming they had been underpaid and overworked.

The company has rejected suggestions of misconduct or corruption over the contract.

The audit is expected to be tabled in parliament in January 2020.

Record number of visa applications

Senate estimates also heard the department expects to finalise a record 9.7 million visa applications this financial year.

Mr Pezzullo said the number of applications made each year had risen by one million compared to three years ago.

The number of refusals and appeals against those decisions is also up.

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"Assessing applications against more complicated and targeted risk profiles has led to more visa refusals," Mr Pezzullo told the committee.

The number of cases awaiting review by the Administrative Appeals Tribunal has more than doubled from 21,404 in February 2017 to more than 55,500 in Febuary this year.

Mr Pezzullo said the majority of refusals were upheld by the tribunal.

Additional reporting by AAP