The Department of Corrections only learnt about a riot at Waikeria Prison after they were contacted by the media yesterday afternoon.

It's believed the riot could have started due to prisoners having a lack of access to phones but Corrections had received no pre-warnings or formal complaints.

The stand-off continues at the Waikato prison, with inmates believed to now have full control of a wing.

Corrections chief executive Jeremy Lightfoot says they received the call at midday yesterday.

A short time later several fires were lit in an exercise yard but not all 20 prisoners in the yard at the time took part in the riot.

In total, there were 21 prisoners who were non-compliant in the top jail area. Four gave up overnight and 17 remain non-compliant.

No food has been supplied to the prisoners on the roof since they begun the riot.

Lightfoot says he would not be sending any of his staff into an unsafe environment, saying "we must be focused on the threat to life."

Substantial damage to the top jail has been "significant" and it is unlikely prisoners will be able to be accommodated there again.

Lightfoot said the prison is losing roughly a third of the prison's capacity - 250 beds - as a result of the fire damage.

Corrections will not reveal any strategies about how they plan to get the prisoners off the roof but said they were working closely with police negotiators.

Lightfoot would not say which method they were using to communicate with inmates.

"Theses are incidents that we train for and are prepared for."

There are 45 Fire and Emergency New Zealand staff on scene and at the height of the blaze there were 75.

There were concerns about smoke inhalation for prison staff but no one was treated for any injuries, Lightfood says.

The exercise yards have shower doors which may have been used as weapons in the riot, Lightfood says.

The most important stage of the riot response was removing prisoners not involved and staff away from the area safely.

He said it would be "very surprising" if rumours of a culture of bullying at the prison were true.

Prison Corrections Association union rep Alan Whitley said the section of the prison is far beyond is used by date.

One of the prisoner staff managers said guards works in "horrible" conditions in that older section of the facility.

The guards are "pretty nervy" working there today, he said.