Home Affairs boss Mike Pezzullo has defended his department's advice to reopen Christmas Island, as the bill rises to nearly $27 million. It has housed just four people in that time, Biloela family.
The offshore detention centre was moved into "hot contingency" in February, in response to the passage of the Medevac legislation.
It brought on a testy exchange between Mr Pezzullo and Greens Senator Nick McKim, during which the secretary tried to shoot down criticisms by declaring "we're the professionals".
Head of Operation Sovereign Borders Major General Craig Furini said that as of Monday morning, 341 applications to come to Australia had been made under the Medevac laws, of which 293 had been deemed valid.
But so far, the only detainees on Christmas Island were the four members of the Biloela family, as they wait to hear the outcome of a long-running appeal over their refugee status.
A minimum of about 100 staff was required to keep the detention centre running, officials told Senate estimates on Monday evening.
The cost had reached $26.8 million, as of August 31.
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Greens senator Nick McKim sad the decision to recommission Christmas Island was "grossly flawed".
"Apparently we were going to get a flood of boats which never happened," Senator McKim said.
"It's obviously been a surprise to the department that those things didn't happen because that was the department's advice to Mr Dutton.
"It was a terrible decision based on totally flawed advice from your department Mr Pezzullo."
Mr Pezzullo said the "precautionary option" to reopen Christmas Island had been advised in response to the original version of the Medevac bill, which he described as "catastrophic".
"The risk to the deterrent value that had been hard-won over those six years [since Operation Sovereign Borders began] was such that in December last year ... when the catastrophic version of this legislation was first presented ... that was our assessment," Mr Pezzullo said.
Senator McKim asked, "You were wrong weren't you?" .
Mr Pezzullo said, "In terms of the version that passed the house that had some moderated measures it was still the judgement of the department ... it still required as a precautionary measure the activation of that contingency measure. That was before the Biloela family were on the horizon, that was a completely different track of consideration.
"And it remains the department's view that along with a constellation of other measures ... we have shored up the risk presented to operation sovereign borders by the Medevac legislation."
Senator McKim interjected, "Or there was no risk."
Mr Pezzullo hit back, "We're the professionals in this field and we judge that there were."