The tax cost ratepayers $26 and businesses $91, on average, this financial year, raising $2 million.

The money is being spent on a project that includes improvements to the lake and ecosystems.

A council report said the tax would reduce environmental threats, leading to savings of $75 million.

Cr Garry Edwards said he was "not clear how we could quantify $75 million in cold hard cash".

Council sustainability manager Dr Quentin Espey said it was an estimate made using "the best environmental economic tools available".

After Dr Espey's second attempt to describe the estimate, Cr Edwards said: "Like the lake, it's becoming a little clearer".

In jest, Mayor Greg Piper said: "It still smells fishy though".