If an employer asks you to obtain an ABN prior to commencement of work, be aware that doing so will mean that you are not an employee in the ordinary sense, but rather a contractor or separate business in your own right.
The practical effect is that you will not receive the same benefits as an employee. Therefore you may not be paid superannuation by the employer, may not be protected under the workers compensation insurance taken out by your employer and should consider obtaining your own insurance policies. In addition, the employer will not withhold tax from your pay, meaning you will have to make your own tax payments to the ATO.
This information is of a general nature only and does not constitute professional advice. You must seek professional advice in relation to your particular circumstances before acting.
Updated — Aug 25, 2015