A business name is the name your business operates under. By law, you need to register a business name if you conduct business under a name other than your own.
It is an offence under section 18 of the Business Names Registration Act (Cth) to carry on a business under an unregistered business name. Failure to adhere to this provision will result in a penalty of 30 penalty units ($3300)
There's a great chance that if you haven’t registered your business name, someone else will. In fact, they may even take it one step further and have secured the intellectual property rights in the name by way of trademark or copyright. If so, you may be liable under the Trade Marks Act 1995(Cth) and the trademark holder can enforce his/her proprietary rights in order to seek an injunction (to stop you from using the name), damages (monetary compensation paid by you) or an account of profits (you hand over all money earned under that name).
Notwithstanding the financial and legal ramifications of trading under an unregistered business name, your industry and client reputation will almost certainly come tumbling down as they may view and your operation as illegitimate.