Apple has quietly switched on turn by turn navigation for Australia in it's Maps app in iOS 6.
I used the feature this morning and it works flawlessly rerouting on the fly if you miss a turn. It will even let you open another app and give you audio cues in the background as well as notifcation banners on the top of your screen.
Simply open your Maps app, type in a location and hit the car icon and your on your way.
It also works great with Siri, simply ask her a question like "Where is the nearest petrol station" and maps will open and tell you how to get there!