Grass carp have been found in Rangitīkei's Dudding Lake, forcing a re-think of how to improve water quality at the camping spot.
Niwa staff surveyed the lake earlier this year and in May experts spent two days hearing about the state of the lake, which is subject to algal blooms.
Horizons Regional Council and Rangitīkei District Council staff have had conflicting ideas about how to restore it.
The lake is the centrepiece of Dudding Lake Motor Camp and Picnic Park, run by a trust and part-funded by Rangitīkei District Council.
For the past six years it has had signs warning against swimming for a few weeks in November-December, due to the presence of toxic algal blooms.
In November, monitoring by Horizons showed the lake was close to "flipping" - changing from a water body dominated by water plants to one dominated by cyanobacteria/bluegreen algae.
"This would likely result in the lake having more frequent and severe potentially toxic cyanobacteria blooms, restricting the lake for contact recreation proposes," Horizons' natural resources and partnerships manager Dr Jon Roygard said.
A "flipped" lake can be fixed, he said, but it's expensive. It's cheaper to prevent that occurring and monitoring was stepped up.
Because of that finding, Horizons Regional Council brought together a panel of experts to discuss the lake on May 22-23.
It involved Niwa, Rangitīkei District Council, the Conservation Department, Public Health and others at a cost of $22,000.
The group came up with possible scenarios to restore the lake, which included catchment-level interventions.
Soon after that the pest fish survey by Niwa was repeated and grass carp were found in the lake, negating one of the possible improvement options.
The next step is to find out how many of the carp are present.
Having them there is not a bad thing, the lake's unpaid operations manager Bruce Gordon - also the chairman of Horizons - said. Introducing them to the lake to eat nuisance weed - as part of a package - figured in two of the three solutions the experts came up with.
"They eat weeds, they can't breed and they can be removed," Gordon said.
Rangitīkei District Council owns the lake and its strategy and planning manager Blair Jamieson had earlier applied to the Conservation Department to release grass carp into the lake, at the rate of 32 per hectare. Other councils use them to control weed, and they are often illegally released.
The council hasn't had a response yet, and the application is ongoing.
Meanwhile, there's a high risk another algal bloom will happen in early summer, closing the lake to swimming again.
One option to prevent this would be dosing it with aluminium sulphate, to lock up the nutrients that feed algae and weeds.
Rangitīkei District Council doesn't favour this, Jamieson said, preferring to use carp and freshwater mussels (kakahi) to clear the water.
The panel of experts still needs to come up with a plan and final report. Gordon is just happy the "argy bargy" is over and people are working together for a solution.