I have to admit that I have not checked in at SYD F desk more than 3 hours before the flight, but I know at MEL I have checked in on-line, and then dropped the baggage off 6 hours before a flight at the F check-in. The possible difference is that at MEL the F check-in luggage is taken manually, i.e. not put onto a conveyor belt, as the F check-in is in a separate area not connected to the main check-in luggage system. The put your luggage on a trolley and take it over for you.

Maybe a phone call will see if SYD will do the same.