I would have thought most plants would be "safe" in Papakura. You don't get that cold up there.
Netting can be useful if you are troubled by cats or falling leaves.
Best way to stop cats is to make sure the edging wobbles or rocks. They do not like it when the ground moves when they go to look in the water.
I doubt water level makes a difference. A friend had trouble with a neighbour's cat who just dived in, swam about until it caught a fish (and these were huge!) then swam out with it :roll: