Fouling of lakes and rivers country's 'largest' environmental concern.

Two scientists speaking at a major freshwater conference have described reversing the fouling of New Zealand's waterways as our largest environmental problem.

The New Zealand Freshwater Sciences Society's annual conference opens in Dunedin today, after a year that has thrust the state of lakes and rivers into the spotlight.

Most recently, New Zealand's "100 per cent Pure" image was challenged when a Ministry for the Environment report found that more than half of our monitored river sites were unsafe to swim in, information later used in a New York Times article on the eve of last week's premiere of The Hobbit.

Professor David Hamilton, the Bay of Plenty Regional Council's lake restoration chairman and biological sciences professor at Waikato University, said the status of New Zealand's freshwater was now "crucial".


"The current rate of intensification and the ability to maintain or enhance our aquatic systems don't match up at the moment," he told the Herald.

"What we've got to do is apply the same level of technology and thinking to the back end, the effluent end, as we have done at the production end. It's as simple as that.

"We've got a very expensive system and we've become leaders in the production side of things in aspects of milking and production, but we are a long way off in regard to the effluent side of things."

Dr Hamilton also believed a bigger focus could be made on preventing nutrient loading into waterways upstream.

Dr Mike Joy, a senior lecturer at Massey University's Environmental Science and Ecology Group, said waterways would continue to deteriorate if the Government did not get tougher on land intensification.

Dr Joy said most of the nutrient-loading stemmed from dairy waste.

"One cow produces 14 times the human equivalent - that means we've got a population of 70 or 80 million, so no wonder we've got a problem."