In order to work in Australia legally you will need to apply for a TFN (Tax File Number). A TFN is a unique number issued to you by the Inland Revenue, which allows you to work and pay taxes!
To apply for your Tax File Number visit http://ato.gov.au/.
Your Tax File Number should take between 2 - 3 weeks to arrive. You do not have to wait for your Tax File Number to arrive before starting any employment, once you have applied you will be given a temporary number.
Anyone holding a Working Holiday Visa is entitled to a tax return, so all or part of the tax they paid whilst working in Australia could be returned!
You can contact the Inland Revenue from within Australia on: 13 28 61 or from outside of Australia on: +61 2 6216 1111 The Inland Revenue is open from 8.30am - 4.45pm Monday to Friday.
Alternatively you can find more information at http://ato.gov.au/.