Australia has spent $30 million and is employing 100 staff to detain just one family on Christmas Island – who are fighting deportation to Sri Lanka.

The extraordinary cost of housing a Tamil family from the central Queensland town of Biloela was outlined in Senate estimates hearings on Monday night, as officials defended the decision to fly the family on a chartered jet to the island.

The decision to detain the family came despite the fact there are 62,000 unlawful non-citizens living like them in the community.

Officials confirmed the reason for this was simply to remove the family from national protests about their plight. The protests had been deemed a safety issue for the family, protesters and police.

During Senate estimates, Greens senator Nick McKim questioned “so we spent in the region of $30 million to detain four people for a couple of months. Is that right?”.

Australian Border Force Commissioner Michael Outram made the point that the facility wasn’t expressly established to detain four people.

“But what you’ve said is factually inaccurate and there are four people there now,” he said.

Mr Outram said national protests, including at Melbourne airport, were the reason to take Priya, her husband Nadesalingam and their Australian-born daughters Kopika, 4, and Tharunicaa, 2 to Christmas Island after the courts slapped an injunction on the family’s deportation.

“None of the detention centres are suitable for children. I have to think about safety and security. This case has been quite emotive.” he said.

“The temporary accommodation [in Darwin], frankly, wasn’t suitable from a safety point of view,” he said.

“Nor did I want to take them to Melbourne, so I took the decision [that] the best place to position them would be Christmas Island.”

Greens Senator Nick McKim repeatedly asked: “Why are they in detention at all? Why are they not at home in Biloela?”

In response, the Home Affairs departmental secretary Mike Pezzullo said pending the outcome of the latest court appeal, the intention was to deport the family.

“They are unlawful non-citizens on a removal pathway,” he said.

Mr Pezzullo added that they had certainly availed themselves of free legal advice they have been given.

The family fears persecution if they return to Sri Lanka. Photo: AAP

In the hearings, Victorian Liberal Senator Sarah Henderson also asked whether Priya and Nadesalingam were told that if they had children in Australia they would also be deported if the family’s refugee claims were found not to be genuine.

Mr Dutton has previously claimed the couple’s two children are “anchor babies” designed to boost their citizenship hopes.

Australia does not offer citizenship by birth but by descent from parents, or by application and conferral.

Mr Pezzullo said several thousand Sri Lankan families who had claimed refugee protection had returned home in recent years.