The Corrections Department is temporarily closing three units at Hawke's Bay prison and may have to transfer up 100 inmates after the discovery of mould in one of the buildings.
The department closed three medium security units in the prison's main block, following the discovery of mould in the roof after heavy rain in July.
Central regional manager Leanne Field says tests showed there is a high level of spores in the mould which could have compromised prisoners' health.
Ms Field says the health and safety of staff and inmates is a priority and it has been decided to temporarily transfer inmates to other jails.
It could be six to eight months before the roof is repaired and the prisoners returned to Hawke's Bay.