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.
+1 votewrong interest rate mate
http://www.ato.gov.au/individuals/content.aspx?menuid=0&doc=...and if you look at how it is calculated, it is simple interest and not compounding daily. ato not really that stupid (trust me i deal with them on a nearly daily basis - they're slow (painfully slow), sneaky, cunning, etc. but not exactly stupid).
Trance N Dance on 23/08/2012 - 18:02 ¶IIRC it's as long as you want, just that you have to keep the proof for deductions for a further 7 years from the time of claiming.

haha…..sorry mate, I had to laugh….a classic mistake.
The table is called - 'Shortfall interest charge'. Its what they CHARGE you. Yes it's higher than the banks. Not surprisingly, it's less than what they PAY you, if you claim a tax refund. (see qazwsx comment above for a link to that table). What they pay you is approximately about what you'd get from the bank.
2
U can do that, from what I understand, back in the day, people did it all the time.