Hi Joe and Janis,
It's MPI Biosecurity, not so much customs that you need to worry about. Customs is more about duty. Biosecurity is about preventing pests and diseases from coming into the country.
Make a list of your gear, take some photos and send an email with the list and photos to
[email protected] for advice on meeting biosecurity requirements.
Generally saltwater gear is not such a problem, the important thing is to ensure all gear is fully CLEANED (very detailed), completely DRY and DECLARED fully on arrival as per your arrival card.
Your items maybe inspected so pack them easily accessible and all together to make it easy for the officers and faster for you. Most likely you will just be asked some questions about your gear and how you use it.
Don't bring felt soled wading boots and make sure any camping and hiking equipment is also clean and dry especially tents and hiking boots.
When cleaning think about things like soil on or inside tent poles or even small seeds stuck to used socks.
Have a browse of the saltwater fly forum section and you will find lots of info on saltwater fly fishing around Auckland and NZ and plenty of help as well if you post questions.
https://fishandgame.org.nz/ is the best starting point for researching trout and salmon fishing, then ask the guys in the freshwater forum for more details. They have detailed maps and downloadable guides to most of the fish-able water around the country.
Hope that helps and all the best for your trip.