I don't know of any in the default FSX NZ scenery. However there are some addons, which model seaplane bases. It is quite easy to create your own water runways - you need just a starting point and a name.
If you search at Avsim FSX: Scenery for rotorua
you will find some of toprob's scenery, which includes a floatplane base but does not provide a starting point. Best thing there is to taxi to a jetty, and save your flight at that point.
Scott Hickey has also modelled the Mechanic's Bay heliport in Auckland, which you can use as a starting point for amphibians. Search Avsim for Auckland
I have these installed. In your search for Auckland, you will also find some seaplane sceneries by Godfrey Tier.. I have not tried these, because they are set for ORBX, which I do not use. Nothing wrong with Goff's work - I used a lot in my FS9.
At Half Moon Bay on Stewart Island, the Widgeons just ran up onto the beach., so you could create a start point there, or as I said earlier, just land, taxi onto the beach, and save your flight at that point.
Sorry to be so slow in answering - I was overseas when I first saw your request, and then I forgot about it.
All the Best