A freshwater ecologist says New Zealanders are not being given a true picture of how dirty the country's rivers and streams are.
Dr Mike Joy of Massey University says a recent NIWA report claimed the levels of bacteria and nitrates at its monitoring sites were generally improving.
But Dr Joy says his own analysis of the science agency's numbers show two-thirds of sites with a significant trend are actually declining in quality.
A report by the Ministry for the Environment has found 61% of swimming holes rated either poor or very poor, which means they are not suitable for swimming.
Dr Joy says new freshwater standards will do nothing to improve water quality, and in some cases may allow it to decline even further, because they set the acceptable limits at the status quo.