A review is taking place at Christchurch Men's Prison after three high-security inmates were able to access yards after lock-up.

Three prisoners in the Miro management unit were able to access individual, secure yards which came off their cells for almost four hours after the prison went into lockdown on Saturday.

Prison Director Jo Harrex says the lockdown started at 4.30pm - but the individual doors weren't closed until around 8.15pm.

She says there was no risk to the public, staff or the security of the prison.

Harrex says they will be carrying out an event review.