The Qantas site has the following information:
http://www.qantas.com.au/info/flying...Guide/sydIntT1
Transferring from T1 (International Airport) to T2 and T3 (Domestic Airports):
After you clear customs, recheck your baggage at the Seamless Transfer Check-in counters and then board the shuttle bus for the Domestic terminal. The shuttle bus service is available to passengers that are travelling or have travelled on a Qantas flight (including codeshare flights) or oneworld flight. It operates from 6am to 12.00noon every 10 minutes and from 12.00noon until 9.40pm every 20 minutes. The bus arrives at T3 opposite Gate 1. Check the screens for departure gate information
Transferring from T2 and T3 (Domestic Airports) to T1 (International Airport):
The Transfer Lounge is located airside (after passing through the security check point), opposite Gate 1 in T3. A shuttle bus service is available to passengers that are travelling or have travelled on a Qantas flight (including codeshare flights) or oneworld flight. The shuttle buses operate every 10-20 minutes from 6am to 9.40pm. On arrival at T1 proceed up the escalator, through immigration and security to your departure gate.