A new discovery from Environment Canterbury vindicates public concern about water contamination from intensive farming.

That's according to Water New Zealand.

Until now it was thought the Waimakariri River acted as a barrier that protected Christchurch's water sources from the effects of intensive farming.

Now Environment Canterbury predicts nitrates from fertilisers could leach through to wells south of the river, in the next few decades.

Water New Zealand CEO John Pfahlert said ECan's realisation has a lot of implications.

"We're going to have to be far more vigilant as a community about the sort of farming practices we have in the hinterlands, surrounding our towns and cities in NZ."

The discovery hasn't surprised the manager of a North Canterbury irrigation scheme.

Waimakariri Irrigation Manager Brent Walton said the report confirms some of his suspicions.

He said leaching often links back to overwatering when people don't put enough thought into irrigation volumes.