Arriving at the airport much earlier than your actual flight time is good advice in general, but thanks to recently introduced "security measures" from the government, the major airlines are now recommending international passengers arrive up to three hours before their scheduled departure times and two hours for domestic trips.
Here's Qantas' message:
The Australian Government has introduced additional aviation security measures at international and domestic terminals at Australia's major airports.
Customers can expect to experience an increased level of security scrutiny at the airport so it may take a little longer than usual to get through the process:
Passengers should arrive at the airport two hours before domestic flights and three hours before international flights to allow ample time for screening.
Limit the amount of carry-on and checked baggage they travel with as this will help to ensure security screening is efficient.
There are no changes to what can and cannot be carried on-board the aircraft.
Thank you for your understanding and patience.
Compared to current recommended times, which range from 45-120 minutes, you're looking at an additional hour on average.
On the one hand, you might get lucky and breeze through security and customs with minimal fuss, but the odds of that happening are, well... let's just say you should probably play it safe and rock up earlier.