The country's most polluted lake is actually full of life, with some fish species flourishing and bird life unchanged.
Lake Ellesmere, near Christchurch, is at the top of the scale used to measure nitrogen and phosphorus pollution, Lincoln University professor Ken Hughey says.
Despite that, its stock of native birds appears unchanged, and while toxic levels of nitrogen have all but wiped out brown trout, eels and flounder appear to be flourishing.
Professor Hughey has told the National Wetlands Conference the shallow nature of the lake and high winds are preventing algal blooms from suffocating life by stirring up the water and refilling it with oxygen.
However he says to return it to a more pristine state will require a massive effort to fence off the wetlands and streams leading into the lake from cattle.