The Victoria University gym! It's low-key and the equipment isn't top notch, but for a non-student I think it costs about $200 a year. You can get free consultations to set up a program and stuff, and they do aerobics classes etc.
The facilities are okay, they have about 15 bikes, 3 steppers, 5 rowing machines and then the usual complement of weights machines and free-weights. It's not usually /too/ busy but varies a bit.
Ignore the tough-guy buff boys and you'll be sweet.