Chaos broke out at Whanganui Prison on Wednesday night with a fire started and a number of windows smashed by inmates.

Corrections said it was investigating how the fire was able to be ignited within a cell and two of the inmates involved were removed from the prison.

The director at Whanganui Prison Reti Pearse, said at about 3am on Thursday a sprinkler on the landing was activated after prisoners managed to "ignite material" in their cells.

The smoke activated the wing sprinklers which immediately doused the fire and the Fire Service attended and reset the alarm and sprinkler system.

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Earlier that same night prison staff responded to a series of banging noises in the wing and found prisonsers had broken windows in three cells.

Pearse said they were observation windows that look onto the prison landing.

There had also been some damage done to the interior of the cells.

After that had been attended to by staff, prisoners then broke the outward facing windows of their cells, which stand behind metal bars.

Pearse assured the inmates remained secure with no risk to any other prisoners or staff and would be dealt with accordingly.

"Six high-security prisoners were involved and have been placed on directed segregation and relocated out of the unit," he said in a statement.

"Two have since been transferred from Whanganui Prison.

"The other four prisoners have been separated.

"A full review of the incident is underway and those involved will be held to account for their actions and the damage they have caused."

Pearse denied it had been an organised riot, which Corrections defined as "a concerted, organised disorder event requiring external agency involvement".