Wait for the big hue and cry from Federated Farmers about Labour's small proposed tax on water use for irrigators etc. Funding needed to clean up waterways.
At the same time, in the Waikato, home to the biggest number of dairy cows, the district council is calling on more famers each year, and finding 23% of them are fully compliant in their dairy effluent treatment, to local body standards. Many others have ponds big enough, but can't prove they are sealed. Others have ponds too small to contain rainstorm conditions. It's expensive to change this, and the council gives them time to do it.
https://www.stuff.co.nz/business/far...15+August+2017
10% of these dairy farmers visited, were clearly too far outside the specs.
Don't forget, dairy effluent is just a small part of the total effluent left on a farm. The rest are point sources spread through every paddock and race, and none of this can be controlled at all. Riparian strips can keep some of it out of waterways nearby, but in many cases the Waikato ground is porous, so there's no escaping the main fact.
More cows lead to more waterways damage under current systems, unless farmers can find a way to contain all effluent or neutralise it on their farms. Maybe barns aren't so bad after all.
Water quality is going to play a part in the elections, and I respect the idea of user-pays. If I had an operation that impacted on the environment, I'd be happy to pay towards my share of that.