I was shocked to learn that my washing machine uses about 150-200l per load, so I did a bit of digging to find out about grey water usage.
I found a fantastic thing that processes and stores grey water so that you can use it whenever and wherever you want. Better than a rain water tank, they only work if it rains!
Check out http://www.perpetualwater.com.au. I thought it was the answer to my prayers, until I found out it costs about $15k....