Are you flying NZ trans-tasman? Available lounges are UA, NZ and SQ. NZ is the best.

Int->int transfer at SYD is easy. On arrival head for international transfer (just before immigration). If UA arrival gate is on the OW pier (it does happen sometimes) then you'll need to take the passage to the *A pier - either on arrival level or departures (follow the signs for high numbered gates for *A pier). There is a security check at transfers then upstairs to departure level. You may need to get BP reissued at transfer desk.

If UA->QF trans-tasman then same as above but head for the low numbered gates. QF has a business class lounge straight ahead and up another level from where the transfers come out.