User blog comment:Spawny0908/Rockstar hints at triple XP tomorrow (Saturday) afternoon (Eastren time)/@comment-3331785-20110326222950/@comment-3455248-20110327033217

Rockstar has said that there will be future sessions specifically geared towards times convenient to Australians. I'm guessing this one wasn't one of them since it was 1 - 3 PM Eastern, which I think means it would have started at 01:00 - 03:00 (AM) in Australia. Since they didn't announce the start time until midnight in Australia (at the earliest), you would have had to be checking their Twitter/Facebook then.

Besides these random sessions, your next official triple xp chance is this Thursday 17:00 - 20:00 Eastern which would be Friday (or Thursday? - forget how the date line works) starting sometime between 05:00 and 07:00 in Australia. For that event they'll have a countdown clock on the Social Club site to help determine the exact time.