Hi! You don't say what part of Australia you're hitting -- it's a huge country/continent, but if you're flying into Sydney, that's a good jump point to hit the Gold Coast between Sydney & Brisbane. From what I hear, the weather is great, the surfing awesome, with plenty of nightlife to round out your day tripping. Check out this link for a cool video on a recent Australian Road Trip.
Keep in mind their summer months are December - February so plan accordingly (Autumn: March to May, Winter: June to August, Spring: September to November).
However, their dry season is from May through October so it's good for road tripping!
You'll have a blast!