Over 3 Million Australians currently use rainwater from tanks for drinking [ABS 1994] in urban and rural regions with no reported epidemics or wide spread adverse health effects. In contrast there have been notable failures of mains water supplies including a viral outbreak in Sunbury Victoria that affected thousands of people and the Sydney Water Cryptosporidium scare that affected almost 4 million people.
The critical factor affecting the quality of tank water is the cleanliness and suitability of the collection surfaces. The collection area should be sealed with a product suitable for collection of rain water and free of contaminants such as lead based paint. The collection area should be inspected regularly to ensure that the area does not have rodent or bird bodies that provide a source of faecal chloroforms
It is essential to fit a suitable filter before using your rainwater tank for drinking purposes.
As an alternative you can boil your water before consumption, this will kill any micro biological organisms but will not remove impurities such as lead contamination.
Should your stored water become contaminated or you suspect it may be contaminated seek professional advice and avoid drinking or using the water in food preparation.
Do not collect drinking water from recently painted roofs (until after the first few rainfalls), timber roofs preserved with chemicals, roofs coated with lead-based or tar-based coatings or parts of roofs near flues from solid wood heaters. Most other roof types will normally be safe for drinking water collection, provided they are kept clean.
Regularly clean your roof and gutter to remove leaves and other debris. It is essential that your down pipes be fitted with a first flush diverter and the input filter on your tank remains fitted at all times and regularly checked for foreign matter debris.
Do not remove the input filter from the tank top, not only are you exposing your water to airborne contaminants but the sunlight on the exposed water surface will promote algae growth.