Yay, someone else from Mangawhai, Maybe I will see you up there occasionally when I am working. :bggrn:
Your pond is probably spring feed, as there is lots of water close to the surface in the foothills of the brederwans. Does your pond flow into a stream or when it overflows does it just run down a paddock?
If it is connected to a stream you could see if there are any eels, feed them, then encourage them to 'hang out' in the pond.
Water movement could help clear up the pond, but if the pond has lots of gunk floating in it any filter on your solar pump could clog up very quickly.
I don't know much about native pond plants, but I do know you want to do everything to make sure you don't get 'alligator weed' in your pond, as it is annoyingly invasive, and very hard to get rid of.