Basically, a .com.au domain name identifies you as an Australian business. This has a number of advantages if you are targeting Australian Internet users.
Probably the most important effect a .com.au domain has is that most of the major search engines, like Google, filter their results according to where the searcher is accessing the internet from. And when a search engine knows that you live in Australia, it will often make the assumption that you will be more interested in Australian websites, and so it may place Australian websites higher up the search results than international ones.
This may not always be the case, but over time you will find that, depending on your product, .com.au domains may rank higher in Australia than international domain names. This is very important if your market is solely in Australia.
However, if your website market is global then a .com address may be more appropriate for you. Many larger corporations who have offices and products on an international scale prefer to keep a .com address so that search engines will display their website on an equal footing right across the globe.
Having said all of that, there is no reason why you cannot have both! In many cases having both a .com and .com.au address pointing to your site can give you a significant advantage from the point of view of your website's market and search engines.