Dudding Lake could "flip" into one dominated by potentially toxic algae within two months to two years, Horizons Regional Council natural resources manager Dr Jon Roygard says.
Its closures are getting more frequent and urgent action is needed as the holiday season approaches.
The popular camping and picnic area between Bulls and Turakina has a warning sign up, because its blue/green algae level is more than twice that recommended for swimming.
Children taking a dip there three weeks ago suffered a stinging allergic rash that lasted two days.
Restoring the lake needs the best expert minds, Roygard said. Possible solutions include clearing the algae from the lake surface, to allow light to reach the weeds growing in the depths. A chemical could also be added, to tie up the phosphate that is feeding algal growth.