Ok, thanks. We've looked at tanks. I will find out whats needed for fire fighting, we knew this would be needed to be taken into account, but the tanks we looked at you can get 45000L tanks for about $6000. $20 000 seems like quite alot? Thats only about 12 000 gallons, but thats massive compared to the rainwater storage most people have.
Where we lived previously thats what we had & we had the biggest tank in town. All the new houses were only putting in about 20 000 litres.
Will look more into that though.