The decision by Immigration Minister Alex Hawke to grant visas to all but the youngest member of the Murragappan family is incomprehensible, their legal representation has said.

The family remain stuck in Perth and unable to return to their Queensland community, despite being granted visas.

The family’s lawyer Carina Ford said Mr Hawke needs to explain why he gave three-month bridging visas to every member of the Murugappan family except four-year-old Tharnicaa.

She remains under a community detention order which requires her to live at a Perth residence nominated by the government.

Ms Ford said the minister’s actions effectively prevent the family from returning to Biloela, the Queensland town that has passionately backed their long battle to remain in Australia.

“I just cannot comprehend why the youngest child should not also be on the same status,” Ms Ford told ABC radio.

“I think the minister probably needs to answer that question.”

She said father Nades, mother Priya, and the couple’s eldest child Kopika, 6, could return to their town under the terms of their bridging visas – but would obviously never do that without Tharnicaa.

“Effectively this ties them to Perth,” she said.

Tweet from @CarinaFordImmi

Mr Hawke said on Wednesday afternoon he had used powers of discretion, allowed under the Migration Act, to step in and grant visas to the family.

“Yesterday, at the request of the Sri Lankan family formerly resident in Christmas Island, I exercised my power under section 195A of the Migration Act, granting members of the family three-month bridging visas, providing work and study rights,” Mr Hawke said in a statement.

“This decision allows three members of the family to reside in the Perth community on bridging visas while the youngest child’s medical care, and the family’s legal matters, are ongoing. The fourth family member’s visa status is unchanged.”

The family was flown to the Western Australian city in early June after Tharnicaa became seriously ill with a blood infection while in detention on Christmas Island.

She has been receiving ongoing treatment since being released from hospital.

• Related: Biloela family rejoice as Tharnicaa released from hospital

But Ms Ford said there is no medical justification for keeping her under a community detention order.

“You can still receive medical care on a bridging visa,” she said.

“I’m hoping that in the coming days that position may change with the minister, because it just seems unfair.”

Tweet from @HometoBilo

A spokesman for the minister said on Thursday that the decision relates to Tharnicaa’s need for medical care.

“The minister made a residence determination given that one family member needs to be in Perth for ongoing medical treatment,” the spokesman said.

The Refugee Council of Australia details the different provisions for people in community detention and those on bridging visas.

People who are in community detention “are generally provided support by a contracted non-governmental organisation”, according to the organisation’s website. They receive “housing and essentials”.

Meanwhile those on bridging visa are not provided with government housing, but are allowed to work and study – and are not subjected to curfews.

-with AAP