Catch and release is fine (in practice, I would say that if they haven't come into contact with fish/water from another river system/established aquarium/etc and it's the same day you should be fine).
A 2m yank would be wonderful for a native setup, would allow for a wide variety of flow. That would also be an ideal size for some big kokopu if you find that natives take you fancy.
To start with I would stock with bullies to start with, you have giant; common; Cran's; upland; and bluegill in your area I think, then during the whitebait run get some live ones (either catch them or buy them from a friendly baiter) and add them in the spring. You never know what you'll get as whitebait, would be bound to get inanga and could well get banded kokopu and koaro. All three of these would do well.
Another interesting addition could be some torrentfish - although they take a bit to get used to catching.
Bluegill bullies, redfin bullies, torrentfish and koaro all like good flow so if you get any of these then a wavemaker or 2 placed near the bottom pointing along the tank would be a good idea (I have a 5-15000 l/h and a 5000 l/h in my 430 l native aquarium). Inanga, kokopu (except shortjaw) common & Cran's bullies, smelt etc don't need too much flow but will often sit in the current anyway.
If you have anything else to ask, ask away...
Edit: have a read through mu 430l tank log (see sig) and dig up some of Stella's tanks as well for some good ideas