There are prisoners on the roof and in the yard at Waikeria Prison in Waikato who have not been contained, the president of the Corrections officers' union says.
A small group of prisoners at Waikeria prison have been rioting this evening after setting fires in an exercise yard, according to Corrections.
In a statement, a Corrections spokesperson said officers were negotiating with nine prisoners who were "refusing to comply with instructions from staff and damaging prison property".
Fire and Emergency this afternoon said firefighters had been called to the prison this afternoon to help guards put out fires in the exercise yard.
They extinguished the fires and remained onsite.
A spokesperson said the prison had been locked down as a precaution.
Corrections Association president Alan Whitley said some prisoners on the roof and in the yard had not been contained.
He said the union was offering support.
"We're always concerned about people when a situation like this is going on, but we've got specialised teams that have got specialised training, they're professional people and they'll do a professional job to get the situation under control."
Corrections said there were 20 prisoners using the yard at the time of the fires. A spokesperson said the "several small fires" had been lit by the prisoners in the exercise yard they were in, but several were "not taking part in the incident".
"We have comprehensive procedures for managing events of prisoner disorder. Our Advanced Control & Restraint teams with specialist capability are on standby to assist if necessary. Our top priority is to resolve the situation safely," the statement said.
The prison remained locked down about 5.45pm.
They said there was no threat to public safety.
Waikeria, one of the country's largest prisons with more than 750 inmates, is near Te Awamutu in Waikato.