Millions of litres of Canterbury's water may have been taken illegally, as the extent of non-compliance among high-scale water users is revealed.

Data shows that in one year, consented users were recorded taking hundreds of millions of litres of water above their entitlements.

They received few fines and there were no prosecutions. The vast majority were not punished at all.

The exact volume of water taken cannot be known, because 500 large-scale users are still not measuring their takes, despite national requirements to do so.

Environment Canterbury (ECan) has been urged to take a harder stance on the small minority breaking the rules.

It was reported in March that about 350 monitored water-use consents in Canterbury – almost one in five of those monitored – were significantly non-compliant.

While the vast majority of irrigators followed the rules, official information from ECan's records, released to advocacy group Forest & Bird, showed the extent of non-compliance by those who didn't.

The data, collected between 2013 and 2014, revealed some users recorded taking hundreds of millions of litres of water more than they were entitled to, with numerous others either reporting inaccurate data or not recording data at all.

Water was being taken during low-flow restrictions, in areas suffering under drought-like conditions with rivers running dry.

Others had broken rules several seasons in a row, ignoring threats of enforcement action.

"These are serious environmental crimes that are punishable by fines and prison sentences under the Resource Management Act, yet ECan seems unwilling to do much more than issue abatement notices, which it then fails to enforce," said Forest & Bird senior legal counsel Peter Anderson.

“Elsewhere in New Zealand abatement notices are taken very seriously. However, in Canterbury irrigators seem to be able to be ignore them with impunity.”

In one example, a Mackenzie district farm had been caught taking too much water three years in a row.

The compliance officer told the farm that ECan was "taking the matter very seriously" due to the farm's "very poor" compliance history, but only issued an abatement notice – effectively a warning.

Another farm recorded illegally taking 31 million litres of water over 42 days, almost entirely during low-flow restrictions. Another was given an abatement notice after not reporting data for nearly two years.

Some had large data gaps, where recording stopped for weeks at a time. ECan said that while 91 per cent of large water users now had meters installed, 500 still didn't, nearly six years after the requirement to do so was first announced.

"There is simply no excuse for taking the water for months or even years before installing a water meter,” Anderson said.

Farmers have reported issues with data providers and the accuracy of water usage measuring. It means usage on a property can be over-reported and the error only discovered months later.

For the past few years, the regional council had focused on ensuring water users measured their takes correctly, said ECan principal resource management advisor Richard Purdon.

It was a complex task due to the sheer number of consents in Canterbury – but now that had been largely done, it could turn its focus to demanding compliance.

"Given the scale of the task in Canterbury, this has been a huge job – it is now largely in place and our focus is on dealing with the small number of people who have not complied, as well as encouraging water use and nutrient efficiency.

"Our experience is that once consent holders know what is expected of them, they want to comply and take action."

He said the 500 people still not measuring their takes had all been contacted and warned they faced enforcement action.

The evidential threshold for prosecution was high, he said, which was why no-one had been prosecuted for exceeding their takes.

Many will have to report this season's data in the coming weeks, which Purdon said ECan planned to examine closely.

"We have a comprehensive plan to scrutinise this information for compliance and to take enforcement action against those who continue to flaunt the metering regulations and their resource consent limits."