The full bench of Australia’s High Court will rule on Wednesday on the legality of offshore detention in foreign countries — the centrepiece of both Labor and Liberal border protection policies.

The test case is the plight of a Bangladeshi pregnant asylum seeker brought to Australia for medical treatment after being held on Christmas Island and Nauru.

Her lawyers at the Human Rights Law Centre argue it was illegal for the Australian government to operate and pay for offshore detention in a third country.

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“The legality is complex but the morality is simple – it would be fundamentally wrong for the government to condemn these families to a life in limbo on Nauru,” spokesman Daniel Webb said.

Because of this argument, the High Court ruling may apply not just to her case, but to Australia’s entire offshore detention program, although it is unclear if a finding in favour of the Bangladeshi asylum seeker would force an end to the program.

Its legality was challenged unsuccessfully in 2014, when lawyers argued the government had no legal power to detain the refugees. This test case is the first time lawyers have argued that the government cannot legally fund offshore detention.

The federal government, with Labor’s support, retrospectively changed laws in an attempt to shore up its ability to pay for the offshore facilities.

If the government wins the case, the female asylum seeker and more than 267 others brought to Australia for medical treatment could be returned to Nauru.

Asylum seekers on Nauru were expected to hold a protest rally on the island to coincide with the court ruling.

-with AAP and ABC