docbert

For QF, it's a WILL be able to use the bus - regardless of who you are departing on as long as you arrive into SYD on QF then you can use it - but be sure to keep your inbound boarding pass as you will likely need it (normally they ask for baggage tags, but with only carryon luggage you obviously won't have any). However calling it an "airside" bus is a little generous - it does leave from airside, and it does drive through the airport, but it then drops you landside at international, so other than the fact it avoids traffic by going airside there's no real benefit to it being an airside bus.

For VA, I'll agree it's a may. The few times I've done it on multiple tickets I've been able to get a ticket to use it, but I've always had to ask, and it's never really been clear if they were just doing me a favor or not. As you said, this one is landside and it drives along the normal public streets between the airports, so it can take longer than the QF bus, especially if there's traffic.