AIRwin, the JQ flight, with a J cabin, ie, tag flights, needs you to have an orange colour label/sticker on your JQ BP, as a dom pax on an int flight...
so (MEL int - SYD int terminal), even if you dep MEL int terminal, your passport in MEL is not put into the outgoing smartgate reader for outgoing immi, they instead mumble while "looking for their outgoing" to stamp the orange label/sticker on the BP.
The BP is also collected off you in SYD as you come through where the smartgate tix is normally taken off you.
SO your idea to stay in the int terminal, on the secure side, to bypass outgoing immi in SYD T1 won't work.
You will have to go through incoming immi, ie, having your orange label/sticker BP stamped in SYD, as in incoming dom pax on an int flight, and then go through outgoing immi where your passport is processed through outgoing smartgate.
(((***UNLESS the whole orange colour label/sticker system on BP is no longer in use***))), its a pain in the bum way to stay in the secure zone to save time, or to use the lounge longer.