Not a tetrapod, obviously, but fascinating creatures nonetheless. I couldn't make a 100% positive identification, what with the murky water, and the eels always being on the move, but I'm pretty sure these were shortfin eels, Anguilla australis. These were at Western Springs Park in Auckland, where I got a few tets recently. They are one of 3 species living in New Zealand, the others being the endemic longfin eel, A. dieffenbachii, and the recently arrived Australian or speckled longfin eel, A. reinhardtii.
Like all members of the genus, the shortfin eel is catadromous, living most of its life in freshwater, then returning to the sea to spawn. Where exactly they spawn is not known, but it is believed to be somewhere in the deep tropical seas north of New Zealand. It is not restricted to New Zealand, but also lives in Australia, on oceanic islands such as Norfolk and Lord Howe Island, and on New Caledonia.
The New Zealand longfin eel is the one of the largest eels in the world, reaching over 2m and 40+ kg, though such specimens are rare these days. They were even featured on the Animal Planet show River Monsters, as stories exist of them attacking people, particularly during the settlement of New Zealand, when there would have been more exceptionally large individuals about. I've embedded below a video of someone feeding some large longfin eels, and there are plenty more videos on youtube if you care to search for them. The shortfin eels at Western Springs Park are also accustomed to being fed, though most of them would be about 50cm long, and you could encircle their bodies with a thumb and forefinger. It does mean they are very inquisitive though, and whenever I put my camera in the water they would come up and put their snouts on the lens.