Domestic to International Transfer in Sydney Between QF and SQ

I need to fly to SIN for business and based on where I'll be coming from will be flying into SYD on Qantas, then needing to transfer to SYD international for my SQ flight to SIN.

Would I be able to use Qantas' bus transfer, or exit the terminal and get the bus/train?

In my experience it's hit and miss depending who is on the door.

I've done similar to this many, many times....flying from Canberra to Sydney on Qantas and then a non-aligned airline out of Sydney to the international destination. In reality it'll depend on whether or not your bag will get checked through from the original departure point. If the staff won't check it through then you'll need to pick it up at domestic baggage collection and then schlep it over the international terminal, meaning you won't be able to use the Qantas bus transfer. But....of late I've taken to just taking the train between terminals. Costs only a few dollars, takes credit card "tap and go" and trains run typically every few minutes. It's a simple process and takes about the same time as the Qantas bus or can actually be quicker (with that bus seemingly only running every 20 minutes or so most of the time).

General rule is if both flights on the same ticket (i.e. QF and SQ) Qantas will let you use the transfer bus, if you don't have a boarding pass (for the Non-QF Flight) just show your baggage receipt with the two flights, if you don't have a bag give them your booking ref they can check if its been ticketed correctly and let you on the bus.

Totally agree with @ChristopherSolo. With luggage (or even without luggage), just take the train between the two terminals. I've always felt it's quicker and less stressful than waiting for the bus, and then the crowded bus ride itself.

Thanks everyone. Have booked so let's see how I go.