I've never had any problems using either the Qantas or Virgin transfer buses for international -> domestic when arriving into SYD on separate tickets on a different airline (UA).

Note that the Virgin transfer area entrance is *before* you leave the secure area - after passing through customs you turn left right before you go through the door to the main arrivals hall.

If you miss it you can also get there from the main arrivals hall, but that involves making your way through the mess that the arrivals hall often is.