Australian Border Force officials have defended taking a Sri Lankan family from their central Queensland home and placing them in immigration detention 1,800km away.

Tamils Priya and Nadesalingam and their two Australian-born children were swept up in a dawn raid of their Biloela home in early March. The couple are in detention in Melbourne with their two girls, three-year-old Kopika and baby Tharunicaa, as they await a decision on whether they will be deported.

The ABF commissioner, Michael Outram, was quizzed about the case during a Senate estimates hearing in Canberra on Monday.

Asked why the family was taken in the raid, he said: “Presumably because they were determined to be unlawful non-citizens.

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“Under the Migration Act, if someone is unlawful, an unlawful non-citizen, our officers are obliged under the act to detain them.”

The Greens senator Nick McKim repeatedly asked the commissioner why the family was treated differently from backpackers sent reminder text messages asking them to leave after overstaying their visas.

“Well, because we don’t have 64,600 officers to go and detain all the people who are unlawful at any point in time,” Outram said. “The backpacker who gets a nudge and goes home, that is not this case.”

The commissioner said the family had been through a series of legal processes and exhausted their right to remain in Australia.

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“If in those circumstances someone is unlawful, irrespective of their individual circumstance, we’re obligated to detain them, with dignity of course and appropriately,” he said. “But that’s our job. As professional law enforcement officers, we have to do that dispassionately but we have to do it.”

The ABF assistant commissioner, Vanessa Holben, said the most appropriate facility for the family was in Melbourne, not Brisbane.

“That facility can provide them with the requirements of their health, their medical needs and their family support with each other, and also schooling and education,” Holben said.