Would suggest setting up Google analytics tracking on the .com.au (it is easy to do let me know if you need help), i.e. redirect to the .net.au but track the origin of sales. If you have some degree of repeat visitors and people navigating directly to do the domain it is pretty much guaranteed there will be some sales coming in via the .
com.au. Personally I would do this before moving the site so you can see the confusion that would have been previously occurring.
Agree with what Shane is saying though about doing the move, obviously it would need a bit of research on the best way to do it without losing too much traffic.