Upvote:4
JQ1 (Sydney to Melbourne), QF19 on Thursdays (to Brisbane) for a start.
How? Australian Business Travellers has a post on this very topic:
How to get into the Qantas First Class Lounge on domestic flights
The catch is knowing which of Jetstar's many Sydney-Melbourne flights to book.
When you're browsing the Jetstar timetable online, let your mouse hover over the small information icon next to the flight duration. A pop-up flight box will show details of the flight, including the departure terminal.
Most Jetstar flights will depart from 'Sydney Domestic Terminal T2' or 'Melbourne Tullamarine Domestic Terminal T1', but the ones you want will instead be flagged as departing from Sydney's International Terminal T1 or Melbourne Tullamarine's International Terminal T2.