I'm fascinated to find out how many people on here rely on rainwater only for their supplies.  I'm one of the lucky ones who has a well and haven't had to worry about having enough water.  I don't know if you're living here yet Coldalba or just planning ahead ?
The rainfall here is erratic to say the least.  It's not unusual to have not a drop from say April to September and then when it does come it's an almighty torrent.  The average annual rainfall is about 600mm which equates to 600 litres/sqm of catchment area.  Now I estimate that I use about 500 litres a day for everything except drinking.  I realise that may sound extravagant to some people (but we don't have a shortage).  That comes out at about 182,000 litres a year or 182 cubo's and would require over 300sqm of catchment.   That's a lot of storage space and catchment area.  Then when you've collected all of that you've got to try and keep it sweet.  If I'm talking rubbish, please let me know.  :o