Studies show rainwater collected from roofs, far from being pure, mostly fails to meet drinking standards.
Massey University lecturer Stan Abbott said e-coli and faecal coliforms were present in more than half of the rainwater storage tanks monitored.
Cases of campylobacter and salmonella resulting from contaminated rainwater supply had been
recorded, he said, and there was potential for contamination with more harmful pathogens, such as giardia and cryptosporidium.
Mr Abbott, who lectures in microbiology and communicable diseases, said about 380,000 people used roof rainwater.
That number was likely to increase as more people bought lifestyle blocks in rural areas not served by municipal town supplies.
But people settling in the country often neglected basic matters, such as monitoring the water quality, and cleaning tanks, gutters and roofs - mostly through ignorance.
During studies of rainwater supply systems, tanks had been found with holes which let in various pathogens, as well as far larger invaders, such as possums, frogs and ducks, which were found floating.
Even a passing seagull defecating on a house roof could raise the level of faecal coliforms in water and pass on other pathogens.
But Mr Abbott said illness outbreaks attributed to roof water were relatively infrequent.
"The health risks associated with contaminated rainwater consumption are not well defined or quantified and relatively few roof-collected rainwater-linked disease outbreaks have been reported in New Zealand and overseas."
This lack of concrete evidence linking illness and poor quality roof water inhibited moves to improve systems delivering rainwater for consumption.
Another inhibiting factor was the macho "Kiwi-joker syndrome".
New Zealanders, particularly men, often saw cases of the "trots" two or three times a week as a normal function and were unlikely to seek medical help, therefore keeping the problem hidden.
Mr Abbott said people built immunity to bugs, or symptoms, but visitors remained at risk.
- NZPA
