Minister of Education Hekia Parata has revealed 17 schools across New Zealand have used seclusion practices this year.
Ms Parata moved to ban seclusion rooms in early November after finding eight of the country's 64 special schools had used them.
The Chief Ombudsman is also investigating.
RNZ reported this morning that the Sara Cohen School in Dunedin was under investigation for allegations of abusive treatment of pupils and the use of a seclusion room in a laundry.
There was also a risk some schools had not owned up to the practice, with official advice to Ms Parata saying there could be more schools that had not disclosed their use of seclusion.
The Ministry of Education said it relied on schools to self-report the practice, and there remains a risk that some schools did not report it.
Ms Parata said the allegations at Sara Cohen were very serious, which was why the ministry had appointed a limited statutory manager to the school.
She said a survey showed 17 schools - out of 2529 schools - had used seclusion this year, but they had all now stopped
She had asked a select committee to consider adding a law change to make the rooms illegal.
The law would make clear what exactly was banned, as it was still legal to use tools such as sensory rooms and time-out rooms, she said.
The public would also be invited to make submissions.