Correct, you will need an international flight to access the first lounges. In MEL very easy transfer, head through customs and to your left is the Qantas domestic terminal. Recheck bags and go through security to the Business lounge. You can take liquids on Australian domestic flights.

you may also wish to check with the staff at the business check in desk or hanging around the automated checkin desks about an earlier flight to Syd. With intl connections they tend to be more lenient on changes.

QF domestic lounge will have showers, food, internet, coffee, no alcohol before midday.