I would suggest it really depends on what type of girls (or guys) you are into, so let me completely stereotype some venues for you. I find all of these places are decent for meeting other singles..
- Austral (Alternative / down to earth)
- Exchange (Alternative / rockers)
- First - Hotel Richmond in the mall (Attractive / upperclass)
- Gilbert Street Hotel (friendly, girl next door)
- Archer (Attractive, hunt in packs)
- PJs (Mixed bag, 1 night stands)
All these venues should have singles in mid to late 20s and up.
Good luck in your adventures!