Since on the one ticket at LAS they should check the luggage all the way through to SYD. They should be able to give you all boarding passes but if they can't give you the SYD BP, you can collect one at a Qantas lounge entrance (see below) or at the gate. Note that Qantas ordinarily use biometric boarding these days at LAX, matching your mug with your passport, no paper or app barcode required.
You can walk airside from T6 to TBIT. Go down tunnel at T6 and follow the signs to T5/T4 (Do not follow any signs to baggage claim or you may have to to reclear security!). Go past T5 staying in tunnel to T4. Head up escalator at T4, turn right and head down to Gate 41 on left. Follow signs to Tom Bradley (walk past G41 lounge), go up escalator and follow the elevated connector to TBIT.
If you have access to a Qantas lounge, stay on the connector level, do not go down any escalator. At the end of the connector passage there is a right hand turn. The First lounge entrance is immediately there, or if you have business lounge access, proceed ~80 metres to the entrance of the Business lounge.