I came across this offer while researching this product online, that I saw in store. Basically buy a bottle of their bleach in tablet form for under $5, fill out your bank details and answer a couple of questions on the claim form, and mail it with a copy of the receipt before 20/3/2012. They will reimburse your purchase price and 60c postage. Essentially you will get it free for a few minutes of your time (you will be up for the stationery costs :) )
I'd recommend printing out a plain claim form and fill it in by hand, I encountered a number of technical problems trying to type in my details online. As far as I can tell, you can buy it from any retailer. There were none specified in the terms and conditions. I didn't see any promotional material in store at Coles or Woolworths at point-of-sale. I'm assuming there will be no problems if you buy it from the major retailers.
One claim per household.
UPDATE:
Promoter seems to have backflipped on the end date of this promotion, purchases now need to have been made by 24 February 2012 and claims received by 29 February 2012