There's some great diving around Melbourne - some very cool wrecks, and the pier at Portsea is a great little dive.
Where abouts in New Zealand would you be?
I'm obviously biased, but if you're interested in diving and the outdoors then I'd say that New Zealand had more to offer than Ballarat. The other side of the argument, if you're also into the city living then Melbourne beats anything in NZ hands down. I head over the Melbourne fairly regularly to get some good shopping, a bit of culture and some variety.....
Ballarat/Geelong/Melbourne is a long way to get anywhere different - long drives to get to places like the Grampian mountains or along the Great Ocean Road for example. Internal flights in Aus are quite cheap, though, so you could easily get up to Sydney, the Gold Coast or Cairns for a bit of variety.
New Zealand, distances are short - but travel time can be quite long as the roads are shocking at times. Lots of diving variety, from Auckland you can head up to the Poor Knights (in Cousteau's top ten) in around 3 hours, or diving on an active volcano (White Island) in about 4 hours. There's then also wrecks like the Mikhail Lermentov and the joy of black coral as shallow as 20m at some of the Fjordland dive sites.