There are many different ways that everyday people can monitor their waterways. The Whitebait Connection (WBC) uses a mixture of biological (living) and non-biological (non-living) indicators to assess and monitor the health of streams, rivers, lakes and wetlands.
If it's freshwater fish specifically that you want to monitor you could also get data collection templates and upload your information onto the NZ Freshwater Fish Database.
Not a lot is known about Inanga spawning areas in many parts of NZ, so it would be very good to let your local regional council or Department of Conservation office, know if you find any of these important areas. Inanga usually make up around 90% of the Whitebait catch and the destruction of their spawning areas is cited as being largely to blame for their decline.