Pray for Rain and buy a Tank NOW:
Australia is burning, dam levels are falling and water will be contaminated, you are not allowed to water your garden how long do you want to wait to buy that tank? Or get a second bigger one? Water restrictions are tough but they can get worse! Vegetables prices are set to go through the roof.
And: a tank can help to put out a fire.
The simple rule is: the bigger the better! Our tanks fills easily and overflows quite often, we have 20.000 liters connected to an average sized roof. First look at you roof: were are your down pipes? Can they feed into one tank or do you need more? Bigger tanks are usually cheaper per liter of water saved and slimline tanks are usually the more expensive option. Instead of a slimline tank check whether you can fit two round tanks in, you’ll harvest more water for the dollar. In my opinion, 5000 liters are the absolute minimum, but I would opt for way more.
How will you use your water tank?
Of course you can bucket your water, but I really do recommend a pump for watering your garden. You can use the water for your household too, you will need either a pump which automatically switches between tank water and mains water when the tank water is insufficient or a manual switch. We have the first version installed, simply because the plumber didn’t inform us that there was a manual option. Nowadays, I would opt for the manual option. First, everything automatic can break and our pump did break (I was still not told about the manual option). Second, if you have the automatic option, the pump will go off with every tiny leak, a dripping tap for instance. This wears the pump out. Apparently, a pressure accumulator is the solution to this problem, even if you don’t have dripping taps, you would be surprised how often you open a tap. If you want to use tank water for anything inside the house, leaf eaters or first flush diverters are a must and cleaning gutters is a must too (which you should do for fire safety anyway!).
For further reading I recommend the Milkwood permaculture article on different tank materials.
On grey water use, I recommend this website: http://oasisdesign.net/