Water from the Waikato River is free of gender-altering chemicals and is safe to drink, says Environment Waikato.
Greens co-leader Jeanette Fitzsimons said the river might be contaminated with chemicals from the urine of women on the contraceptive pill, which can change the sex of fish and lower male fertility.
Shewants work on a pipeline to take Waikato River water to Auckland to be stopped.
British research found that half the male fish in Britain's low-lying rivers were developing female characteristics because of the chemical effects of treated sewage discharged into rivers.
But Environment Waikato said yesterday that it had tested for the chemicals two years ago and cleared the river of any risk.
Environmental contamination scientist Nick Kim said very low levels of the chemicals at Hamilton's sewage outfall into the river had been detected.
The levels were much lower than those found overseas and were massively diluted once they entered the river, he said.
The work found "absolutely no oestrogenic activity" in Hamilton's drinking water.
Asked whether water from the river was safe to drink in light of the British research, he said: "On the basis of what we know, it's just not even on the radar screen as a significant risk."
The research was carried out by Auckland University graduate student Jennifer Gadd, who tested river water for the presence of hormone-disrupting chemicals including those identified by the British research.
The five-year British Environment Agency study showed that the urine of women who took the contraceptive pill contained a form of the hormone oestrogen.
Previous studies showed a wide range of industrial chemicals were changing the fish, but the latest research suggested the main culprit was a synthetic oestrogen called ethanol oestriadol from contraceptive pills.
The Waikato intake for Auckland's drinking water is downstream of treated effluent being discharged from Hamilton, Ngaruawahia and Huntly.
