If your business operates in Australia, then a yourbusinessname.com.au is for you. If you also do business outside of Australia, then a yourbusinessname.com domain should also be purchased. This will protect you from someone purchasing the domain name in the future and at around $15 per year it is a wise investment for your growing business.
What if your domain name is taken? You can always go for the yourbusinessname.net.au equivalent, however the .com.au domains are more professional for a commercial business. Other alternatives could be yourbusinessname.net or yourbusinessname.org.
One great tip would be to consider before purchasing your domain from an SEO point of view is what do you expect people to be typing into the search engines when looking for your products and services?
If your primary business sells fur coats and your business name something completely unrelated like Style City, you may want to look at getting a domain like stylecityfurcoats.com.au. Having keywords that visitors would type into the search engines in the domain will give you a great advantage in terms of search engine optimisation.
If you would like to purchase a domain name check out DomainWave, for low cost Australian Domain Names.