The Sydney change is a bit annoying when going to Brisbane. My experiences:

• Delay arriving at Sydney - QF128 is typically 25 mins EARLY arriving
• Taxi and wait for plane doors to open - 2 mins to 25 mins
• Immigration - 5 mins to 30 mins
• Wait for bags - 1 min to 20 mins
• Clear customs - 1 min to 10 mins (make sure you fill out customs forms on the plane!)
• Bus or train to domestic - 15 mins to 30 mins
• Domestic checkin and bag drop off - 1 min to 10 mins
• Domestic security - 5 mins to 30 mins
• Arrive at gate prior to departure - 10 mins

So I like to say 2.5 hours is safe safe, but normally only need 1hr 30 mins on an average day. When you reach domestic bag drop off at Sydney, the staff are excellent if you've missed your flight and there should be tonnes of flights they can move you to.