Hi,
I assume you mean start a business in the same area as your current employer?
That obviously means you can't take any "company" assets with you (this includes knowledge as well as physical items). You obviously own your experience and your own knowledge - but you can't use specific details about your current customers or your current employer into your new business.
There is nothing illegal about quiting your job today and then going and knocking on your ex employers customers door tomorrow to see if they wish to buy your product or service (but you just need to be very careful).
Even if you do everything 100% above board there is nothing stopping your employer taking you to court - you may win but you may go broke in the process.
If you are serious about this you need to see a lawyer so you know exactly what is and isn't illegal (if you go through a BEC agent -
http://www.becaustralia.org.au/ - they may be able to setup a free first consultation?).
Depending on the situation it may be better to leave and get another job while you work on your new business.
All the best