This depends on what you use your rainwater for. If its outdoor use, you could save an average of 18%. But of course, you could save considerably more if you have a pool, boat or water gardens and lawns frequently.
If you have rainwater plumbed back into your house to wash clothes and flush toilets, this amount can jump to 56%.
You can use rainwater for everything, although this may require additional filtration for hygienic purposes (drinking, showering, dishwashing and so on).
If you plan to connect a tank to your internal plumbing you will require a building consent. Get in touch with your local council for specific information related to your area.