Mostly I'd just visit the airline websites themselves (if they accept Oz Credit cards, or allow Australians to purchase from them).
Most have a "best price" type of guarantee.
Failing that, try expedia.com.au or zuji.com.au or travelocity.com . All will give pretty good prices on N.A flights from the major carriers.
Now if talking low cost airlines, I'm not really sure where you can get comparative prices on them.