While it is not mandatory to provide us with your Tax File Number (TFN) or Australian Business Number (ABN), it is highly recommended to do so. If you choose not to provide your TFN or ABN and have not provided us with an exemption, we will need to withhold tax from your income payments at the highest personal marginal rate plus the Medicare levy.
To avoid this make sure you provide your TFN or ABN as soon as you can. If you have had additional tax withheld due to not providing this please refer to the information here on how you can request this back from the ATO.