The luggage should be checked through to LAX, so
Get off the flight at domestic
head through to arrivals and go across to the train station outside the terminal
Show you QF boarding pass at the station and you should be allowed on for free
Get off at the next stop which will be at the International terminal
Go to passport control and through security and then can head to the QF lounge
There are 2 lounges ; business class and 1st though there isn't a great deal of difference between them

Dave