Computer modelling of what it would take to clean up the Waikato and Waipa rivers and their tributaries has estimated it would cost anywhere between one and nearly eight billion dollars over a 25-year period.
The modelling is based on four scenarios.
The first one would require major changes to land use, and would make the rivers suitable for swimming, fishing and healthy biodiversity.
The next three range from no further water quality decline, but with some improvements, through to just holding the line with no further degradation.
Scenario one would require a 22 percent reduction in dairying and a 27 percent fall in its profits in the region.
The chair of the technical leaders group, which is part of the Collaborative Stakeholders Group conducting the modelling, said the healthy rivers plan was based on reduction in four contaminants - nitrogen, phosporus, sediment and e-coli.
Bryce Cooper said the catchment covered about one million hectares.
"There's lots of development in the catchment. There's lots of economic activitiy in the catchment and there are water quality issues in the catchment. And when you combine all of those things, previous studies have kind of indicated this as well, it is a big challenge and the point of what we are doing here is kind of to say how big is the challenge and then the CSG working through how long it might take to address the water quality issues and what might be the tools and instruments used to do it."
The Collaborative Stakeholders Group chair Bill Wasley said the modelling would be used to help form a draft plan to go to the Waikato Regional Council.
"Assisting to give CSG a feel in terms of what are the implications so they can actually understand then and OK, what may need to be in a plan framework? Mr Wasley said. "And it comes back to the timing, things will not happen overnight in terms of achieving that fishable and swimmable (outcome)."
Mr Wasley said any changes in land-use could take many decades to filter through and improve water quality.