US Apple Store 64GB iPod Touch - $399
AU Apple Store 64GB iPod Touch - $549
So in the US there's a $100 difference between the iPod touch and the iPad.
You are comparing US tax free with AU tax inclusive. It is actually:
US Apple Store 64GB iPod Touch - US$399
AU Apple Store 64GB iPod Touch - AU$499
and that therefore means the AU WiFi 16GB will be $599 by your reckoning
Edit: actually $AU623.75 by your reckoning.