http://www.immi.gov.au/visitors/transit/no-visa.htm
When a visa is not needed
Some travellers do not need a transit visa if they:

a.. depart Australia by air within 8 hours of the scheduled
time of their arrival and
b.. hold confirmed onward booking and documentation necessary
to enter the country of their destination and
c.. remain in the transit lounge at an airport.

I'd get the visa just in case the departure flight is canceled
and the choice becomes sleeping in the transit lounge or in a
hotel room.