Australian Immigration Minister Scott Morrison will have to consider the case of a Tamil Sri Lankan woman in indefinite detention after the High Court ruled his department made a mistake.
In a decision handed down on Thursday, the High Court judges unanimously declared an error of law caused by an invalid regulation meant the woman's visa application was never referred to the minister.
The unnamed woman, referred to as M76/2013, arrived at Christmas Island by boat in 2010.
Because she arrived without a visa, under the Migration Act she could not make a valid application for a visa unless the minister approved.
She was assessed to be a genuine refugee.
But complicating her case, the Australian Security Intelligence Organisation (ASIO) gave an adverse assessment based on her membership of the Sri Lankan Tamil Tigers organisation.
ASIO said she had taken part in armed combat, remained strongly supportive of the group and its aim of achieving a Tamil homeland through violence and was likely to engage in acts prejudicial to Australia's security if granted a visa.
The immigration department took the view that this assessment meant she could not be granted a visa and so her case was never referred for the minister's consideration.
The woman, now married with two children, remains in Villawood detention centre in Sydney.
She can't return to Sri Lanka and despite efforts to find a third country, none will have her.
The High Court judges unanimously held that exercise of the minister's power was affected by the error of law.
As a result, the minister had yet to finalise consideration of her case. (AAP)