Sometimes things just go wrong - it doesn't have to be an earthquake or anything particularly serious, just some old infrastructure finally giving up the ghost, and voila! you have no water for the day.
This is exactly what happened for thousands of Wellington residents in Brooklyn, Mornington & Kingston yesterday. http://www.stuff.co.nz/dominion-post/news/5647128/Wellington-suburbs-lose-water-after-main-bursts
It's great to hear that at least some of them had stored water set aside for just such an event.
Regardless of whether it's for during an emergency, or for when the pipes are bung, you should store at least 3 litres per person in your house per day, and have enough to last three days - so that's at least 9 litres each.
Don't forget, that's just drinking water (you'd be surprised how thirsty you get when you haven't got much to drink), it doesn't include water for cooking, or washing dishes, or getting yourself clean. You should store more water for that if at all possible.
The World Health Organisation has had a fair bit of international experience with disasters and says that 15L of water per person per day is a realistic amount to go through in an emergency. If you're curious, our figures show that you use 350L(!) each per day normally - flushing toilets, doing laundry, having showers & baths, washing the dishes, brushing your teeth, cooking, drinking, and everything else you do with tap water.
Obviously it may be difficult to actually find room to store that amount for water in your house, so just do the best that you can. We've made this a bit easier for you by sourcing some really good 10 litre plastic water containers. They're a bit easier to fit in the closet or under the bed than a dozen Coke bottles, so you can easily make room for two or more for each person in your family.
We've got these bottles for sale at the awesome price of just $5 each from the Porirua City Council offices at 16 Cobham Court.