As Danger mentioned, try that SYD T3 "not well known security screening point" first, its also shorter to get to the QF lounge.
Occasionally, it might not be open, but try it first.
Once you are on the inside of it, you will know where it is.
Its on the same level as arrivals, and quite handy to get to gate 15 too, to get the bus to SYD QFi T1.
===
So, if you see the map below, there are 2 dom security screening points at SYD T3, the main big one with the flight screen that shows all departures, very brightly lit, that is the one everyone uses.
But, to the left of it, you will see a smaller security screening sign, that is the one to aim for.