Yep rainwater! That's not the main house there, it's a relocatable home at the back of the property that his son lives in. I'm guessing it's just easier to run it off tank water than to plumb it into mains. I've got a smaller slimline one on my place though that's about 3000L that i use to top up the pool, water gardens, wash the car with etc. It just gets so hot here that you burn through heaps of water keeping everything alive. Also we go through droughts and water restrictions every couple of years, so it's a necessity if you want to keep a clean car.