Ok, once you are employed there are a number of forms that can be used to change your tax rate, (please note that the Tax File Number Declaration should only be completed once with each employer), the main ones are:

Withholding declaration
This is really only for increasing the tax paid, declaring HECS etc. Claim FTB through a reduction in your tax

Downwards Variation - quote from ATO
2007 PAYG income tax withholding variation (ITWV) application
Approval to reduce the amount you are required to withhold from a payment can be provided only by the Tax Office. This usually happens only in special circumstances where the payee can demonstrate that their final tax liability for all income in the year does not justify the standard withholding rate.
If a downwards variation is approved, we will send an approval letter to the payee and a variation notice to you, outlining the new rate or reduced amount to be withheld. Both the notice and the letter will clearly specify the expiry date of the notice.
Basically they make it easy to pay more tax, but harder to pay less. As usual see your accountant, call the ATO etc if you have any queries.