Man helping detectives after prison assault

A prisoner is helping South West detectives with their investigation into an assault at Bunbury Regional Prison on Sunday morning.

A prisoner was left with serious injuries after the assault at the prison about 9am.

The 40-year-old man is still in a critical but stable condition at Royal Perth Hospital this morning, after he was flown there following the incident between two prisoners.

It is believed the incident occurred at the pre-release centre before guards could intervene.

The assault happened just minutes before visiting hours at the facility.

A 38-year-old man is helping police.