Canterbury Regional Council has agreed to new rules for the Selwyn District and Lake Ellesmere catchment which will increase environmental restrictions on farmers there.
The council has accepted the recommendations from independent commissioners who have been considering changes to the land and water management plan for the area, proposed by the local zone committee.
They are part of measures to clean up Lake Ellesmere, or Te Waihora, which is one of the most polluted lakes in the country.
River and groundwater takes for irrigation will be restricted, and by 2022 farmers will have to reduce nitrogen losses - by up to 30 percent in the case of dairy farms - to improve the water quality of the lake and its catchment area.
The recommendations also set catchment limits for losses of nitrogen from community sewerage and industrial sites.
Environment Canterbury Commissioner Peter Skelton said cleaning up the catchment posed huge challenges.
"That is because there already exists an over-allocation situation in terms of water quantity.
"More water being used than is environmentally available, particularly groundwater and also water quality, in that there are nutrient, in particular nitrogen losses affecting water quality throughout the catchment, and ultimately, of course, in Lake Te Waihora."
Mr Skelton said it was not a quick fix, and that it could take 20 years or more to complete the clean-up process.
Constraints on some Canterbury farmers in new rules
As well as significantly reducing nitrogen and phosphorous discharges under the new rules, most farmers will also require consents to farm.
A Leeston dairy farmer John Sunckell, who is a community representative on the Selwyn Te Waihora zone committee that produced the initial proposals on the rule changes, said it was going to be a fine balancing act, but a necessary one.
"The first thing we have to realise is that we are in a zone that is over-allocated as far as water abstraction goes, so the environmental impacts, particularly in the lowland areas, are quite considerable, and when you match that up with cultural outcomes from Ngai Tahu, it's something that had to happen."
"And on the other side, we are aware with Te Waihora at the bottom of the catchment, that we are over-allocated with nutrients, so we've got a real problem with an over allocation of nitrogen and phosphorus. So what this package does is attempt to pare back from where we were, allow for further development with the Central Plains Water (irrigation scheme) and through that, a re-balance of the water across the catchment," said Mr Sunckell.
"So it is a really fine balancing act to achieve those outcomes that the community demand. And when you spend three or four years looking at the science, you just have to accept there is something to be done."
John Sunckell said it meant farmers would have to have a much better understanding of their farming systems and their environmental impact.
"So a lot more recording, a lot more reporting. We will be required to have farm environment plans to understand our systems and articulate how we will attempt to mediate the environmental effects we are having and unfortunately through the process, just about all of us are going to end up having to have a resource consent to farm."