The issue again remains that the connection DL is selling is a connection on ONE SINGLE TICKET. They are comfortable selling this given the MCT of SYD but also the fact that they are comfortable protecting you should you miss it. On separate tickets there is no protection as I eluded to it can easily take 45 minutes to clear customs in SYD and its not just that, you will have to wait in line to go through the "No Declare" line which can be 15-20 minutes by itself. Even if all this goes well, since you're on separate tickets, you must make your way outside, upstairs, find the DL check in desk, and all of this must happen so you present yourself AT THE DESK before 1 hour prior to departure.

All of the above also assumes your Garuda flight has no Delays.