Risk free interest rate better than the banks
Here me out here as this is a bit convoluted.
You can ammend your tax return at any stage after submitting it to add additional deductions. See http://www.ato.gov.au/content/00314025.htm
The ATO will then pay you any additional amount you are owed, plus the Shortfall interest rate on this additional amount you are owed. The shortfall interest rate is currently 6.66% and appears to be consistently above that offered by the banks (see http://www.ato.gov.au/content/65367.htm)
What is to stop you intentionally not claiming a significant deduction (or deductions), and then claiming it a year later, thus earning a high rate of interest on the differential? Granted this is only going to be worthwhile if you have some big deductions.