That means customs and immigration and transferring to a different terminal (AC #2, QF the TBIT), which will involve taking your bags with you.
That would be true IF the YUL-LAX ticket is issued separately from the LAX-SYD ticket. If Air Canada has issued an interline ticket YUL-LAX-SYD, while the pax will have to go through the customs/immigration stuff to leave the building, if the bags are tagged through to SYD, after going through customs, they can be dropped off with TSA where they'll be rescreened and forwarded to QF.
I've transited through LAX between carriers and changing terminals. On arrival from outside the country have never had to lug my bags that have been checked all the way through to my final destination to the next terminal and check them in again. Pax are directed to drop off bags that have connections and are tagged as such with TSA as they exit immigration/customs.
But, like I said before, IF there are two separately issued tickets involved then lugging bags will have to be done.