Sri Lankan couple Priya and Nades Murugappan and their Australian-born daughters Kopika, four, and Tharunicaa, two, remained in detention on Christmas Island on Saturday awaiting a trial date, which could be months away.

Judge Mordy Bromberg on Thursday extended an injunction preventing the family's removal from Australia until a trial decides the validity of a protection visa application for the youngest girl.

But even if the court permits the application, the subsequent approval remains at the discretion of Home Affairs Minister Peter Dutton - and he's clearly stated the family must leave.

"Ultimately the government may find against the child on the substantive visa claim and I say that because they have made adverse decisions in the case of both parents and sibling," University of Queensland law school associate professor Peter Billings told AAP.

In addition to ruling whether Tharunicaa is eligible to seek protection, a Federal Court judge will also decide if Australia is in breach of its international treaty obligations on torture and civil rights.

But again, the court's finding will only be a recommendation and the government could disagree.

The best hope the family has to remain in Australia is if conditions worsen in Sri Lanka, exposing them to danger if deported.

"If country-of-origin conditions in Sri Lanka deteriorate ... political turmoil, or a spate of terrorist attacks or resurfacing of tensions between Sinhalese and Tamils ... there might be a glimmer of hope," Prof Billings said.

"The government must base their decision on a protection visa based on the conditions (in Sri Lanka) at that time, not backdated to when the application was lodged."

If a protection visa is granted to Tharunicaa, it would expire after three years when the matter would be reassessed and possibly denied.

"Our position is they should leave as soon as possible," Mr Dutton told Sky News on Thursday.

"There are lots of delaying tactics. If there's a case to be made let them make it. We've been very patient with this family."

Judge Bromberg permitted the matter to proceed to trial, finding that lawyers for the girl had established the basis of a legal argument worth pursuing.

"It does buy the Tamil family some more time and it allows them to prosecute the case fully before a Federal Court judge," Prof Billings said.

"But ultimately the minister has to be persuaded there is a valid reason for lifting the statutory bar to grant a visa and that's what he's said will not happen."

Mr Dutton has confirmed that the family will not be separated.

Australian Associated Press