If you rely on water from a rainwater tank, you may be interested to read the following article from the CSIRO which includes tips to keep your tank water healthy.
The following links from articles published in Organic NZ also have excellent articles about ensuring that rain water harvested from roofs is safe to drink:
These are very comprehensive articles that are important reading for anyone dependent on water from their roof.
Ed note: Organic NZ is published by the Soil and Health Association which has been dedicated to supporting organic home gardening and commercial organic agriculture and horticulture in New Zealand for seventy five years. If you are not already familiar with this magazine I highly recommend it.