I would suggest you review the road rules online
http://163.189.7.150/rulesregulation...roadrules.html

especially if you aren't familiar with round-abouts and other road configurations that aren't common in the US.

Other points as some have previously mentioned:

*No turn on red unless there is a specific sign allowing it.
*No U-turn at traffic lights unless specific sign allowing it.
*Speed limits are just that in Australia - there isn't the large leeway you see in the states. Being even 1 Km over the limit can get you fined.
*Keep left.
*Like the rest of the world everything is in KM etc so actually watch your speed indicator until you have a real idea of what speed you are going at.
*Watch for school zones if you are driving in suburban areas.

Otherwise, Australia is as normal to drive in as any first world country.

Oh, and if you are using a rental make sure you have booked an automatic unless you are very comfortably with a manual (stick) transmission.