Since we hooked the tanks up to the guttering, 77 mm of rain has fallen, which has filled the first tank to 3/4 of its height. Given that the tanks are 22700 litres in capacity, that means the 77mm rainfall has given us 17062 litres of water. So, each mm of rain falling on our roof gives us approx 221 litres of water (not a bad deal). Our annual water consumption is 195,803 liters (well it was last year). Annual average rainfall in this area is 1793 mm, which should offer us a potential 396,253 litres per annum. Theoretically, that is: at times, the tanks will overflow. But in general, we should have enough storage, unless we hit a very dry spell.
Here below is an image of the impact of the last 77mm of rainfall on our local river, the Wilson River at Eltham. Not unusually, it's risen by about 3 metres.