Nine Greenpeace protesters have been arrested for trespassing at an irrigation dam west of Christchurch.

Early this morning they occupied the inside of the dam, which has yet to be filled with water, and set up a makeshift camp.

Police moved in and arrested them after an eight-hour standoff.

Greenpeace said it had no choice but to take the action after the dam’s owner, Central Plains Water, threatened it with a court injunction aimed at shutting down a peaceful protest it had planned.

It said it wanted to raise awareness of what it said was widespread public opposition to irrigation schemes on the Canterbury Plains.