Asylum-seeker transfers to the Australian-run Manus Island detention centre in Papua New Guinea could be temporarily halted from tomorrow, when the controversial scheme is tested in the courts.
Lawyers representing the opposition will ask the Peter O'Neill government to stop the transfers until the court makes its ruling.
Loani Henaos, who is bringing the challenge on behalf of the opposition leader Belden Namah, says he expects the government to seek an adjournment.
And he says he will ask the government not to bring in any more asylum seekers until the substantive hearing.
There are currently 274 detainees at the temporary Manus facility - including more than 30 children - living in conditions that have been widely criticised as inhumane.
The Attorney-General Kerenga Kua has argued the site is legal under the nation's immigration law, which grants power to the immigration minister to set up a processing facility.
But Mr Henaos says the agreement with Australia allows the PNG government to bring in asylum seekers from a foreign country, and the minute they put their foot on PNG soil, they are arrested.
He says the opposition claims anyone entering the country has his or her personal liberty guaranteed under the constitution.