A case will be filed in the High Court today challenging the constitutionality of the Rudd government’s Papua New Guinea asylum seeker resettlement plan. The case could have massive ramifications; if successful, it could completely invalidate Australia’s offshore processing of refugees.

Lawyers acting on behalf of an Iranian asylum seeker who is challenging the government’s PNG asylum seeker deal have revealed to Crikey they are expanding and refiling the case today. The man, identified as S156 of 2013 (the High Court number given to him when proceedings commenced last week), claimed refugee status when he arrived in Australia by boat. He was soon sent to PNG against his will. His lawyers want him returned to Australia so his refugee application can proceed “in the normal course”.

The High Court challenge was announced on Tuesday last week, but now lawyers say they are expanding the case to claim the government did not have the power under the constitution to make the decision to send refugees to PNG.

The challenge hinges upon the constitutionality of a declaration signed by then-immigration minister Chris Bowen in 2012 to send asylum seekers to Manus Island in Papua New Guinea. Bowen signed the declaration in the wake of a United Nations High Commissioner report condemning the conditions at Manus, and it is this declaration that formed the basis of the government’s plan to send all asylum seekers arriving by boat to PNG.

Sydney-based immigration solicitor Adrian Joel, acting on behalf of the asylum seeker, told Crikey last night “we’re just going through the motions of refiling and expanding the case”. The expanded case will include a “constitutional issue”, Joel said.

“What that means in plain English is … that there wasn’t power to send the people offshore because the minister didn’t have the power to go through the processes with regards to [the notice] to send people overseas. Because we’re saying the power doesn’t evolve from the constitution. We’re going to assert that,” he said.

Brisbane barrister Stephen Keim SC, who is not working on the case, says this argument “suggests that the power [in the constitution] to make laws with regards to aliens doesn’t extend to making laws like this”. “It’s an argument that says to make laws with regards to a regional processing country in this way is not something the Commonwealth government has power to do under the constitution,” he said.

If successful, the High Court challenge could completely invalidate Australia’s offshore processing of refugees. “If that argument were successful, and I can’t guess at the details of the argument, then it would have very strong implications for offshore processing,” Keim said.

On Monday last week, the Attorney-General’s office issued a statement defending the legality of the PNG plan:

“The government is very confident in the legal basis for its transfers to Papua New Guinea. It will vigorously defend any challenge to the regional processing arrangements with Papua New Guinea.”

In a second extension of the High Court case, Joel says lawyers will look at “whether the correct procedures were adopted in the decision-making, regarding the notice — which is the source of everything — the notice to have New Guinea as the country that people could be sent to”.

In the case lawyers will question “whether the procedures adopted for the identification of countries for refugees to be sent to indicated the correct and appropriate considerations, given the fact that the United Nations indicated that there wasn’t any real way that such assessment could be undertaken in New Guinea”.

Mark Robinson QC, the barrister acting for the asylum seeker, previously told the ABC’s PM program there were factors Bowen had failed to take into account in October last year.

“The minister expressly said that he’s not going to take into account issues relating to the domestic law of PNG or the international obligations of PNG,” he said. “Now, those things are plainly relevant, and it’s incredible to believe that he can make a lawful decision without regards to those factors … Given the fact that the United Nations indicated that there wasn’t any real way that such assessment could be undertaken in New Guinea.”

The United Nations refugee agency detailed five major concerns about the federal government’s plan to send asylum seekers to PNG’s Manus Island in a letter to Bowen, tabled in federal Parliament in October last year. The scathing letter lists PNG’s failure to sign international treaties against torture and for the protection of stateless people, as well as the absence of any national legal or regulatory framework to address refugee issues as reasons it “would not have any operational or active role to play in their implementation”, referring to the offshore processing.