For Ads, if its a once off, and it won't be cheap, and you are worried about the time, I would get a taxi/cab.
Both the train ($5) and the free bus, will run to a schedule, and not fast.
I would say the taxi fare would be about $15/$20, yes, even for that short a distance, as they have to cover their airport charges, you can try
www.taxifare.com.au for a rought idea.
I paid $35, yep, and thats Aud from SYD T3 to SYDT1, early one morning, had to ring up the taxi phone number, as there were no cabs to be seen at 5am, just had walked from the Ibisbudget at SYD dom, thinking there would be cabs that early outside T3, but none in sight.
So I had to pay pay phone booking fee, and also airport taxi charges, for 4.5 to 5km ride, $$$.
Great if you make it onto the train/bus at BNE dom, but who knows, it might wait for time scheduling.
The train (when I use it) at BNE dom sometimes waits there for 10 mins, as does the orange bus.