Immigration Minister Peter Dutton says preparations are underway to accommodate more asylum seekers at Christmas Island in case the Government loses a High Court case against offshore detention.

Lawyers have argued detaining asylum seekers on Nauru is in breach of Australian laws.

Key points: Federal Government plans to house more asylum seekers at Christmas Island, before High Court case verdict

Federal Government plans to house more asylum seekers at Christmas Island, before High Court case verdict Lawyers argue detaining asylum seekers on Nauru is in breach of Australian laws

Lawyers argue detaining asylum seekers on Nauru is in breach of Australian laws Government planning to accommodate more asylum seekers in case of a loss

They have told the High Court that the Federal Government is in control of the centre, despite its operating under Nauruan law.

Mr Dutton said he was confident that the Federal Government was in a strong legal position.

He told Sky News that the High Court was expected to deliver its finding next year, but the Department of Immigration and Border Protection had already started planning in case of a loss.

"It's prudent for my department ... to put in place contingency arrangements," he said.

"If we needed to accommodate more people on Christmas Island, there is some prep work that is taking place there now."

Human Rights Law Centre legal advocacy director and one of the lawyers involved in the case, Daniel Webb, said a win for their client could have significant implications for the Federal Government.

But Mr Webb told the ABC he did not know whether a win would result in an end to offshore detention.

"The questions involved in this case, about the lawfulness of our government's role in detaining people in other countries, they really strike at the core of the current offshore arrangements," he said.

"The answers to those questions, it is fair to say, will have significant implications going forward."

Mr Webb said he was not surprised by the Government's actions on Christmas Island, citing previous measures such as the transition to an open centre on Nauru on the eve of the court hearing.

While the High Court finishes sitting this week, and will not return until February, Mr Webb said a finding could be delivered at any time.

Further detail has been sought from the Department of Immigration and Border Protection.