Pullock, why not take the train? It's gotta be cheaper than a cab, even if you only get it into the city to go to a cab rank near the station.
The train service in Brissie goes every 20 minutes, and is $9. It's pretty good. If you're a pilot working at Brissie (or for that matter any airport worker) you can get a $10 weekly ticket. I often use it when I take the car in for a service than catch the train to and from work for that day. Not sure if the Sydney service is as good.