Some quick comments for you (as an Accountant).
As a sole proprietor, the tax non commercial loss regulations may affect your ability to claim any initial losses against other employment income in the year of loss (Ideally you will make profits from day one so this may not be an issue). Profits will be taxed at marginal rates but as a micro business you may be eligible for concessions with the Entrepreneurial Tax Offset (ETO). This is the simplest, cheapest and possibly most tax friendly option if you are talking small dollars in ancillary business income (and you have average income elsewhere). An ABN is easy to get and can be done online at
www.abr.gov.au (you can also attach a GST registration if appropriate as well - that’s another issue. I would avoid unless necessary).
If your operations are intended to be larger and you are more certain of profits and direction then starting a discretionary trust may be the best option (again it is very dependent on your personal financial position so this is general advice only). You can always roll in a business that you previously started as a sole proprietor so perhaps start off simple and cheap and go from there. Provided the real capital value of the business is minimal at the point of transfer then there is unlikely to be any capital gains tax issue.
If your business concept involves the provision of services then the tax Personal Services Income (PSI) regulations may apply. In effect these can attribute all the income to you personally anyway which makes starting another entity a waste of time and money (from a tax perspective). In short these are intended to apply to individuals that provide services through an entity that otherwise are like those of an employee. ie for personal reward or effort.
One other note - your employment contract with your current employer may regulate any other income earning activities (including businesses and other employment). There may be no regulation at all or it may be a simple case of getting written or verbal consent before you start something "on the side". If your business concept is aligned with your current employment then it’s more likely they will have a problem with it (for many reasons). The last thing you want is an employee running a business via email and phone during regular business hours - I’ve seen it go pear shaped many times (from both sides of the coin). Be respectful of your employer and the agreement that you have made with them. If you build a big business one day then I’m sure you would want the same from your staff.
Hope this helps and best of luck with it.
Regards
James
Full time work - Small business on the side 
