The detention centre was reopened in response to the passage of the so-called Medevac laws, which allow asylum seekers on Manus Island and Nauru to be transferred to Australia more easily for medical treatment.

Prime Minister Scott Morrison said the facility had to be resurrected to deter other refugees from travelling by boat to Australia.

But not one refugee has been transferred to the detention centre since it reopened in March.

During a Senate inquiry into the government's bid to repeal the Medevac laws on Monday, Labor senator Kim Carr quizzed Home Affairs boss Mike Pezzullo about the point of the exercise.

"There was a big song and dance about ... there was a claim at one point this was going to cost $1 billion to reopen Christmas Island. What's all that about?" Senator Carr said.

"You've made reference to singing and dancing. I don't deal with that," Mr Pezzullo said.

"The Prime Minister did a remarkable extravaganza press conference I call rightly during the election campaign. How many actual folks are now on Christmas Island as a result of these measures?" Senator Carr said.

While Mr Pezzullo conceded none, he said the original version of the Medevac bill - which would have allowed future asylum seekers to be transferred to Australia - would have meant the detention centre would have had to be used again.

"If the original version, I think the version I described as the catastrophic version of the legislation, had ever seen the light of day ... the sort of cost involved in resuming our operations on Christmas Island to the tempo and intensity we had previously seen [would have been] $1 billion," Mr Pezzullo said.

Instead the centre was in "hot contingency", immigration detention group manager Kayleen Zakharoff told the committee.

"But it's empty?" Senator Carr asked.

"Yeah. Yep," Ms Zakharoff said.

"Right. So they're guarding an empty facility?" Senator Carr asked.

"Well, they're maintaining its operational status," Ms Zakharoff said.

"It's a real Yes Minister exercise isn't it? And they run much better without people in them don't they? Like hospitals without patients?" Senator Carr said.

"Are you suggesting we send people there?" Mr Pezzullo chimed in.

"No, no, given it's going to cost us an estimated billion dollars," Senator Carr said.

Meanwhile it remains unclear if and when the detention centre will actually close.

In the April budget, it was revealed the government planned to close the centre by July if it won the May election, with only $185 million allocated for its operation.

That announcement came only weeks after the government argued its reopening was urgently required.

It also came seven months after the centre was quietly closed for the first time in a decade.

The Department of Home Affairs was asked whether the planned closure was still on the cards, and when. No response was forthcoming.