This time of year, we think about lack of water and extra water bills. Would having your own rainwater tank be wise or affordable?  Doing the numbers  An average kiwi homes use 150,000 litres a year. There is usually a fixed charge for being in the system then a usage charge for the number of litres you use.  In Auckland that’s $190 fixed and about $500 usage.   If you got a 5000L tank, this would be refilled every time it rains. Estimate this saving at least half your water use or around $250 per annum. A 5000L tank can be as cheap as $950. Or top of the line fancy slim tank that fits anywhere or maybe an in ground tank, $3500 
So in 4-10 years (water prices are only going up) you have paid for it and you are saving that amount of money every year.  
How to collect rainwater? 
My favourite way is to a system that collects water for a while, catching pollution and bird dropping sfrom the roof. You then ditch this, and the cleaner water is sent to your tank 
If you don’t trust the rain water then it can be directed to toilets, washing machines and  garden, or just put a water filter in the system.   Check your council bylaws about your rights here. I know Auckland city has just made it a lot less bureaucratic to install one. They need the help! Often all they ask for is a backflow prevention device to keep your water out of the common supply.  If you are just considering some extra water for your garden then you can go much smaller and simpler and cheaper of course.  It'll save your council money too 
If everyone did this it would considerably lower the city infra structure costs: piped water in and out of your property. That means lower rates for all?  You also reduce the stormwater runoff which in storm periods can mean the grey water invades the sewage system and this ends up in our public waterways and beaches.