UPDATED 2.33PM: Millions of litres of Canterbury's water has been stolen by water users flouting consent rules with irrigation systems.

Forest and Bird information shows between 2013-2014 there were more than 370 instances where irrigators were in serious breach of water consents.

None of them were fined or prosecuted by Environment Canterbury (ECan).

In one instance, one person illegally took 31 million litres of water from a creek over 42 days and only received an abatement notice.

Senior legal counsel Peter Anderson says it's extremely concerning farmers have been allowed to engage in brazen and repeat offending.

"A farmer took much water one year and got told he was taking too much for three or four years and ECan still didn't take any prosecution," Anderson said.

"The seriousness of some of the breaches, combined with the inaction by ECan - people will be very surprised that ECan doesn't seem to be doing its job by enforcing the consent issues properly."

ECAN accepts more needs to be done to make sure farmers follow the rules. CEO Bill Bayfield admits the organisation has struggled in the past, and will "accept the criticism that we haven't done."

"It's been difficult and we've been on a steep learning curve since two-thousand-and-twelve, but I believe over the next few years we're going to crack this," he said.

The Green Party's environment spokesperson Eugenie Sage argues ECan needs to stand up and take control.

"The illegal talking of water could be a lot worse than what is currently reported because ECan don't know the extent of consent breaches because it's not doing the monitoring," she said.

"ECan needs to be much more on the ball. Its relaxed approach to both monitoring and enforcement has seen numerous instances of stock being allowed in waterways and it's contributing to the long-term degradation of our rivers."