So a few comments from a dairy farmer:
*So many variables. Canterbury and Wairarapa are drier soil types. River flow is less so higher chance of pollutants to build up.
In Taranaki however we started a fencing and riparian scheme voluntarily in 1995 - most farmers brought into it and over 97 percent fenced and over 90 ish planted. Canterbury and Wairarapa are further behind.
Stocking rates is also a concern (Cows per hectare) - Taranaki is 2-2.5 cow per hectare and Canterbury is 2.5-3.5. Also Higher input farms in Canterbury and Wairarapa - reliant on brought in feeds etc to maintain higher stock rates. Also depends on where waterways are etc - Most farmland in taranaki is off the mountain shorter stretch then to the sea - less chance of pollutants to build up. Wairarapa is from lake Wairarapa which is fed further up by rivers that have thousands of hectares of farmland and towns cities polluting. Canterbury Has the most pristine water coming from the alps but it meanders slowly so sedimentation happens quicker which will catch nitrates etc. Way more to go into and way more science needed to solve the issues.*