They are as profitable as .co.uk's which sell pretty high. If you are going to buy domains such as those then you should buy something which is copyrighted (be careful) and then offer the companey an offer. Could make a little bit of a profit. Being from Australia myself, I don't really think they are that profitable. Think about it, would you type in xbox360.net.au over xbox360.com?
I think the domain name you register for .com.au or .net.au has to be an exact match, abbreviation or acronym of the applicant's commercial name or trade mark.
I own a few .com.au's only because a few clients wanted them. I think businesses would like the .com.au (if they are from Australia) because if makes them look authentic