i know a couple, but they're online at all times of the day i've noticed; not just when it's light out down under.
If you phrase this question differently non australians can answer too :) Activity slows down between 2 AM EST and like 11 AM EST (on weekdays, especially.) Otherwise there tend to be quite a few ppl online. So that's 5 PM - 2 AM in Sydney. Peak hours are maybe 6pm EST - 12am EST? 9 am - 3pm in Sydney?