The Coalition government’s efforts to head off a High Court challenge contradict its assurances that detaining asylum seekers on Nauru is legal, according to the team running the case.

Labor and the Coalition are set to rush legislation though parliament today after the Labor Caucus agreed to help the Abbott government patch up a hole in Australian law, which could mean the bipartisan policy of offshore detention is illegal.

The Human Rights Law Centre is currently running a case in the High Court on behalf of a group of asylum seekers and their families.

Director of Legal Advocacy Daniel Webb issued a statement this afternoon, saying if the government was confident its actions were lawful there would be no need for it to “suddenly change the law when its actions are challenged in court”.

“Our case asks whether government has the necessary legal authority to run offshore detention centres and to spend billions of taxpayer dollars for that purpose,” Webb said.

“All governments should observe and respect limits on their power, not hastily remove them.”

Webb said the legal team would have to look at the details of the legislation.

After news of the legislation broke, Greens Senator Sarah Hanson-Young asked Attorney-General George Brandis whether he would admit the policy of detaining asylum seekers on Nauru for the past three years was illegal.

“No, that’s incorrect Senator,” Brandis said.

“The government is of the view that the offshore processing arrangements are legal. That is the view of the government.”

Brandis went on to refer to the government’s efforts “to stop the flood of illegal immigrants”, provoking Greens Senator Peter Whish-Wilson to object to the description of asylum seekers as “illegal immigrants” on a point of order. He was unsuccessful.

The government’s own statistics have consistently shown that the overwhelming majority of asylum seekers processed offshore are found to be refugees in need of – and legally entitled to – protection.

A recent report initiated by the Department of Immigration found credible evidence of sexual assaults in the Nauru detention centre, and an ongoing senate inquiry revealed allegations have continued since the report was completed.

In a press conference, Hanson-Young made what appears to be a forlorn call for Labor to vote against the legislation introduced by the Coalition today, calling it a “big moment” for the party.

“Here is an opportunity to make a difference, here is the opportunity to take a stand… do not give the Abbott government unfettered authority and power to keep children detained in the hell-hole prison on Nauru,” Hanson-Young said.

According to the Department of Immigration’s most recent statistics, 634 people remain detained on Nauru, including 81 children.

Today is not the first time the Coalition has introduced legislation to undermine potential legal challenges to Australia’s detention policies.

A wide-ranging bill passed in December last year gave the government explicit power to send babies born in Australia to Nauru, heading off an ongoing legal challenge from Maurice Blackburn, who argued that the stateless children should not be sent offshore.

The new laws were opposed by Labor but backed by key crossbenchers and scuttled the case.

The government also received a stern rebuke from a Victorian Court in May when an officer prevented lawyers inspecting facilities on Christmas Island, in spite of a Court order allowing them access.

The laws introduced today are expected to pass the Senate by Thursday at the latest.

The government is reportedly concerned the High Court could rule against it before parliament returns after its winter recess.

But the Human Rights Law Centre rejected that claim, and said there was no need to rush the legislation through this week.

“People affected by this case include newborn babies, people with serious medical issues and women who report being sexually assaulted on Nauru,” Webb said.

“They have suffered great hardship at the hands of successive Australian governments. They deserve to have the lawfulness of their treatment considered by our courts, not have the government shift the goalposts mid-case.”