In terms of time - school holidays
In terms of cost - it depends on guests, but I've never really been convinced to go to Supanova, so take that as a guide as to how much I'd spend. Probably less if it involves travel to Melbourne.
In terms of guests, I'm not really fussed, as my itch is scratched by getting the read the reports of international guests on the international sites. Maybe people specific to the Australian section of the franchise?