A test case before the High Court in Canberra today will challenge the whole basis of offshore processing for asylum seekers.

Lawyers for a group of Sri Lankan Tamils, refused asylum in Australia, are claiming the Federal Government's processing procedures on Christmas Island are unlawful and unconstitutional.

The Refugee and Immigration Legal Centre's executive director and principal solicitor, David Manne, says the challenge is about fairness.

"Our clients are Sri Lankan Tamils who've fled from their homeland in fear of further brutality on the basis of their race and political beliefs, and they're challenging decisions to refuse them refugee status that were made under the Government's offshore processing regime that's carried out on Christmas Island," he said.

"The Government's position is that under its offshore processing regime, unlike on the mainland, a court can't look at whether the decisions on these life or death matters were made fairly or lawfully.

"But our clients are also saying that in this context that a fair and lawful process should be applied in the same way that it is for people who arrive on the mainland. That they should essentially be on equal footing."

Mr Manne says the Commonwealth will vigorously defend its law as it stands.

"That's quite clear but there's also an additional point in one of the cases and that is that the whole process, the whole legislative scheme, is unconstitutional in the sense that it tries to give the minister a power, a discretionary power, with no duty to consider whether or not he should allow someone to apply, under law, for a protection visa as a refugee," he said.

"Or, in other words, that a court, in this context, can't order a minister to consider whether or not someone should be considered for protection under the Refugees Convention, a treaty that we've signed up to."

He says the unconstitutional nature of how some asylum seekers are refused refugee status dates back to the Howard-government era.

"This whole scheme was originally set up under the former Howard government and has essentially been carried over by the current government with some revision," he said.