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.

Horizons' chairman Bruce Gordon is also the operations manager for the reserve.

He would prefer to leave Rangitīkei District Council to deal with the problem by introducing freshwater mussels to filter the water and nonbreeding carp to eat the weed.

"I'm pleading with you. I really beg Jon to talk to these people doing the work."

He said the lake has had higher algae levels than this in previous years, and always recovered.

Councillor John Barrow asked whether the council had to adhere to current public health standards.

"If it flips we are going to look stupid because we have done nothing," he said.

Roygard said those standards had to be used. He backed the council team to be the best freshwater scientists, and said introducing carp would have other bad effects.


Horizons CEO Michael McCartney said the two councils would talk about the lake. Any immediate action would be at the lower level, and be communicated to councillors.

It would be wrong for Gordon's views to be given more weight than the views of his own council's scientists, councillors Nicola Patrick and Rachel Keedwell said.

The Horizons Region has 220 lakes. They have some "very real issues" and only 15 are monitored, Roygard said.

Algae levels at Lake Wiritoa, near Whanganui, also jumped to twice the level recommended for swimming this week.