Waikawa river has high levels of E. Coli, unlike the Waikawa Reserve site about 10 Km upstream. Between there and here the river flows through cow paddocks and past the shag colony. Now interesting research by Environmental Science and Research suggests that native trees could help.
Native trees could help clean up lakes and rivers and provide a solution to New Zealand's nitrogen and effluent problem.
Previous tests have shown E coli died off much faster under mānuka than under pasture, and significantly reduced the leaching of nitrate compared with pine trees and grass. …
Science leader of the ESR's biowaste team, Maria Gutierrez-Gines, said they did not know how it worked yet, but they thought the native root systems may release compounds that pathogens did not like, so they either died or did not grow.
Native plants put under microscope for E coli and effluent treatment | RNZ News
So there's something to keep in mind.