UPDATED 2.05PM: One of the inmates accused of murdering a fellow prisoner says he's guilty of manslaughter, not murder.

Benton Parata died five days after he was attacked in his high-security cell at Christchurch Men's prison last March.

Akuhuta Tihi, Stephen Betham and Levi Reuben are charged with his murder at the High Court at Christchurch.

In her opening address, Crown prosecutor Deidre Orchard said the trio had been acting to a pre-arranged plan, and were seen entering his cell on CCTV.

Defence counsel Phil Shamy said Tihi admitted to assaulting Parata, but denied having any intent to murder him.

No opening addresses were made on behalf of the other two defendants.

The trial is expected to take three weeks.