The immigration department has provided an undertaking not to send 26 Australian-born babies or their immediate family members to offshore detention, pending the outcome of a court case to determine their legal status.

Maurice Blackburn principal Jacob Varghese said he received the written undertaking on Wednesday evening after threatening legal action if the department did not make such a commitment.

The law firm represents 24 families in detention on Christmas Island and two in Melbourne, who were concerned that they would be removed, like others before them, to offshore detention centres with little notice at night.

Pregnant asylum seekers in offshore detention are typically brought to the mainland to give birth but changes to the Migration Act last year mean that people who arrived in Australia by boat after July 19, 2013 are classed ‘‘unauthorised maritime arrivals’’ and can be removed offshore. The law is silent on asylum seekers’ babies who are born in Australia.

Mr Varghese said the facilities on Christmas Island were ‘‘not suitable’’ for babies, particularly newborns, but parents feared the conditions on Nauru or Manus Island would be worse.

He welcomed the department’s commitment not to move the babies or their families until at least 24 hours after the Federal Circuit Court determined the case of baby Farouz, who was born in Brisbane. A date has not been set for that hearing.

‘‘I’m pleased because it gives these families a degree of piece of mind, at least for the next little while, that they are not going to get that knock on the door in the middle of the night and be whisked off to Nauru,’’ Mr Varghese said.

‘‘From that perspective these 26 families can sleep easier at night.’’

Mr Varghese said the undertaking related only to the 26 families his law firm represented, and he did not know how many other Australian-born babies could be at risk of being moved to offshore detention centres.

‘‘It would just be good to hear Scott Morrison say that no babies are going to be transferred offshore, for all those people we don’t represent,’’ he said.

‘‘But at this stage we are very pleased to have achieved this outcome for our clients and our focus now will be on winning the baby Farouz case.’’

The Greens party is seeking a conscience vote banning the removal of Australian-born babies to offshore detention centres.