Lillies of course. Myriophyllum aquaticum (parrot's feather) does so well in cold water it infests farm dams and drinking troughs.
Papyrus grows well in pond margins, growing in shallow water.
The lotus, Nelumbo spp, will do well but needs to be lifted over winter (lillies are supposed to be lifted and divided too).
Oxygen weed, Elodea canadensis, also grows well. It is the only one of 3 species commonly called 'oxygen weed' that is permitted in aquaria.
There is a plant called alligator weed, Alternanthera philoxeroides, which is a serious problem in waterways north of Auckland.
Baby tears, Bacopa, will also adapt to pond life.
Another to look for is Starwort, Callitriche stagnalis. It is common in streams and/or creeping over mud.
Cardamine, Cardamine lyrata, is best grown as a bog plant and is easily over-run by algae. Commonly called bittercress.
Chara globularis can be found throughout NZ.
Elatine gratioloides is found in shallow margins of large lakes throughout NZ.
Of course, duckweed grows well
Lilaeopsis ruthiana and Lilaeopsis novae-zelandiae are also found in shallow lakes throughout NZ.
Give Lysimachia a go too. It is found in most pet shops (usually under "bunched plants").
Watercress is found in drains and streams. Fish love the roots so it should be regarded as a renewable fishfood rather than a decorative plant
Nitella, stonewort, is another plant to try.
Irises are easy to get hold of and provide lots of colour.
That should give you something to go on for a start :lol: