The Greens say the government has let Cantabrians down environmentally, and they're paddling down the Selwyn River to prove it.

Greens co-leader Russell Norman and water spokesperson Eugenie Sage are taking kayaks down New Zealand's most polluted river, to protest government approved irrigation consents.

Dr Norman says the consents will allow for more cattle around the river, leading to further water degradation.

"The amount of pollution that the National Party's going to put into the lake and into the Selwyn River will be vastly overwhelm any efforts to clean them up."

Russell Norman wants New Zealanders to take a stand and demand cleaner waterways.