No space for a whapping big rainwater tank in the back yard but keen as mustard to harvest the rainwater from your roof run off?  Than check out the eco sac.  The eco sac is like a large flexible water bladder that fits under your house.  Water stored in the eco sac is suitable for safely fill swimming pools and fish ponds without fear of contamination from the bladder.  Eco sac configurations can store from 2,600 litres up to 50,000 litres and qualifies for government rebates where they apply.

Check out the Eco Sac website for details.