If you are travelling F with Qantas, and you have your correct contact details in your booking, then you should get a phonecall the day before your flight from the First Hosts.
They'll tell you a bit about the check-in process, and ask you if you want a spa booking etc.
Otherwise, bookings are available (space permitting) at the lounge on the day.
Both Melbourne and Sydney F lounges have direct phone numbers. I don't know them, but Qantas does publish the following as the numbers for the Intl QP's in SYD/MEL:
(02) 9691 6040
(03) 8336 4294
I assume the numbers above would connect you through to the F lounge if requested.