Analysis of Canterbury's high country lakes shows most have become more polluted over the last 10 years.
All but one of the 14 lakes monitored by the Canterbury Regional Council have degraded since 2005.
Lakes deemed 'clear and blue' in 2005 were now classed as having moderate levels of algae and nutrients.
Only one, Lake Emma, had improved, and only one, Coleridge, was classed as 'very clean'.
The regional council's surface water science manager, Tim Davie, said the pollution could be in the form of nutrients from intensified land use, or other contaminants like faeces.
Meanwhile, in the Bay of Plenty Regional Council's latest study, also released this week, only three out of 12 lakes have shown improvement in the last year.
The council has attributed the poor result to last summer's hot weather.