Direct flights into Queenstown from Oz are daily over the season. I'd wait until July and see what the season/conditions are doing then start planning a trip for mid August. Accomodation is plentiful and packages from Australia are often cheaper than what it costs us to ski even when already in New Zealand.
Ruapehu with 2 ski fields in the North Island usually has the longest season but is very hit and miss for days/lifts open and depends on the snow conditions whether you can even ski terrain outside the normally groomed areas. Stuff you guys say is rubbish at home we would think is probably okay at Ruapehu.
Best bet is Queenstown though with 4 ski fields close and plenty to do otherwise if you only have a limited timeframe. A guided trip out of Wanaka out the the back of Treble Cone or even some heli from QT would be the ultimate depending on budget.
Canterbury fields out of Christchurch are a bit more hard core and basic as said above but trickier to organise and get to and maybe a bit more weather dependant?