Inmates who have been in a tense standoff with guards at Waikeria Prison are now believed to have full control of the wing.
The Herald understands all compliant inmates have been removed from the wing where 16 prisoners have been lighting fires and throwing projectiles at guards who are unable to get close enough to bring the standover to an end.
The prison remains in lockdown today, nearly 24 hours after a group of inmates rebelled and started lighting fires yesterday afternoon.
It is understood most of the wing is now badly fire damaged.
Corrections said this morning that four of 19 inmates had surrounded overnight, but the jail has been significantly damaged and is unlikely to be used to house prisoners again.
The drama started yesterday in the prison's exercise yard around 2pm, and then moved to the roof, where inmates set fire to mattresses.
A source told the Herald that the uprising began after inmates had been denied haircuts, but Corrections could not confirm that until they had talked to the "affected prisoners".
More information is expected to be released about the situation at a press conference this afternoon.
Corrections said the 16 prisoners remained within a secure perimeter on the roof of one building, called the "top jail unit".
One source told the Herald the prisoners had been throwing projectiles at prison guards - wood, metal and anything they could rip off the damaged roof.
As a result, it had been too dangerous for police to get too close to the prisoners. No one had been hurt so far.
The source said there was little risk of the prisoners escaping as there are two perimeter fences - electric and wire - and the fences were being patrolled by Corrections staff.
The weather is hot and sunny, so the source said it was hoped the prisoners would get "hot and hungry" and give themselves up.
Corrections said there had been no loss of life or injury to staff or prisoners. Fire and Emergency NZ, Police and St John remain at the prison and are working closely with Corrections staff.
A Corrections spokesperson said the damaged section of the building was still burning hours after the fires began. It was not safe to attempt to quell the blaze while the rioting continued, he said.
"At this stage the fire hasn't been extinguished. Obviously we need to wait for the non-compliant prisoners to be brought under control before we can send fire and emergency in to fully assess the situation and bring it under control," he said.
"We're unsure how extensive the fire is at this stage but parts are on fire, including mattresses that were set on fire," he said.
A faint stream of smoke can still be seen floating off from one end of the prison facility.
While the condition of the building will need to be assessed, it was unlikely that prisoners will be accommodated there again, Corrections said in a statement. The facility was built in 1911 and was being replaced by a new prison scheduled to open there in 2022.
Corrections described the prisoners behind the revolt as "non-compliant".
Corrections Minister Kelvin Davis did not want to comment while the situation at the prison remained unresolved.
Late yesterday, 49 prisoners were evacuated from the rioting sector facility to another unit in the prison. Overnight a further 163 men were temporarily moved to other locations within the prison, said the spokesperson.
A stream of prisoner vans with blacked out windows continue to exit the facility this afternoon.
Specialist Advanced Control and Restraint teams, made up of staff from a range of prisons, are at the jail, where inmates have been out of control since early Tuesday afternoon.
Photographs emailed to NZME late last night showed a large cloud of smoke visible from neighbouring farmland.
Police, firefighters and St John confirmed they were still onsite as Corrections officers continued to try to negotiate with the inmates and ensure the safety of everyone at the prison.
A spokesperson said yesterday that 19 prisoners had been seen on the roof of the building. This included those who were involved in lighting fires in a yard earlier today, along with several others who were able to get out of their cells.
"The prisoners have been able to access some parts of the building by breaching the roof space, however their movement inside the building is restricted by internal gates, barriers and secured doors.
"There is a large amount of smoke around the building, which is coming from mattresses set alight by the prisoners."
"There are around 230 prisoners in total in the 'top jail' facility and we will not hesitate to evacuate further prisoners if it becomes necessary in order to keep them safe."
FENZ was originally called to Waikeria after inmates lit several fires in the prison's exercise yard on Tuesday afternoon.
About 20 prisoners were using the yard at the time.
The situation was thought to be under control before nine prisoners refused to comply with instructions, Newshub reported.
Offenders allegedly took toilet doors off their hinges and used them as weapons against staff.
Corrections Association president Alan Whitley 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," Whitley told RNZ.
St John treated a number of staff and prisoners due to smoke inhalation. Earlier in the evening it was thought at least one prisoner was bleeding after an altercation with guards.
A prisoner on remand said a riot at the prison had been imminent, with prisoners protesting for basic human rights. They claimed there were issues at the prison, including toilet paper taking days to be provided, Newshub reported.
Last year, two Waikeria Prison Correction officers were punched in the face within nearly as many days, while fighting between prisoners has also been reported.
A prisoner punched an officer in the face and another officer was also injured when he stepped in to help.
That altercation came after an officer was threatened and punched a few days earlier.
There had also been reports beforehand of prisoners fighting among themselves.
Waikeria Prison is one of New Zealand's biggest prisons, located on a 1200ha site south of Te Awamutu in the Waikato region.
The "top jail" where the inmates are currently contained was built in 1911 and is the oldest part of the prison. It is being replaced by a new facility being built at the prison and which is scheduled to open in 2022.