Greenpeace protesters who spent the day fighting an irrigation scheme being built in Canterbury say their protest has attracted more supporters.

The environmental activists spent the day chained to machinery and locked inside pipes at the Central Plains Water Irrigation scheme construction site near Hororata.

They believe the irrigation scheme, which is part-funded by a government loan, will mean more cows and greater pollution of rivers and drinking water.

Central Plains Water Scheme spokesman Derek Crombie said the scheme was protecting ground aquifers because farmers would not have to use wells and artestian supplies.

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Building is set to finish in September 2018.

But construction was halted today when police blocked access to the site for safety reasons.

Protesters headed home this evening after temperatures fell to near freezing.

Campaigner Gen Toop said the protest was "just the start".

"We're putting big irrigation on notice. We will be back, real soon and next time we're inviting the public to join in."