Bought a used 1000L water tank on eBay this week for $235 delivered. $0.23 per litre is cheap and it is just a perfect size to fit under the house. The photo shows the tank being pushed under the house.
I've positioned it so that the top of the tank is level with the overflow level on the existing barrels I have collecting rainwater (you can see one in the photo). So I only have to hook them up and they all act as one tank.
These tanks feed into a small automatic pressure pump which feeds a garden tap and a sprinkler via a solenoid controlled from the home control system. I put a float switch in one barrel just below the overflow level. This is also wired to the home control system so that when the tanks are full a garden sprinkler is turned on for five minutes. The idea is that if we do not have the capacity to store the water we might as well divert it onto the garden. Hence dry spots in the garden get a higher monthly rainfall. Since Melbourne gets rain every few weeks in summer I recon this will help the garden a lot.