Jailers have cut inmates' fruit rations after prisoners were found stockpiling produce to make alcohol.

The Corrections Department said rotting fruit could cause a health hazard for prisoners and staff, and posed a "significant risk".

Inmates in some prisons are now allowed to purchase just five pieces of fruit a week in addition to the mandatory two pieces a day provided by the jailhouse kitchen.

Previously, the quota was 10 apples, 10 oranges and 10 bananas each per week. Jails in New Zealand supply a daily serving of potatoes and two other vegetables, Corrections says.

Prison sources say the fruit crackdown began after the Spring Hill riot in June, when 29 inmates got drunk and went on a 10-hour rampage at the Waikato prison.

Prison homebrew, or "pruno" as it is sometimes called, is commonplace in most jails. It can be made using fruit, water, sugar and a plastic bag.

Prior to the new fruit rules coming into force last week, whole blocks at some northern prisons were locked down so guards could search the cells for fruit contraband, a source said. Prisoners are understood to be less than happy about the changes.

Howard League for Penal Reform spokesman Jarrod Gilbert said the issue was a "double-edged sword . . . It's not in the prison's interest to have alcohol being brewed but it is in the prisoners' interest to have a healthy diet."

Gilbert said the small number of prisoners brewing liquor would have a disproportionate effect on those who just wanted to eat fresh fruit. Servings in prison were not generous and many prisoners felt the need to have extra, he said.

A better solution, he said, would be to give out fruit at intervals to avoid hoarding.

Corrections said the official fruit policy was still under review. It noted there were very few offenders who purchased the maximum amount of fruit each week.