A Northland iwi is appealing to people to stop using a tiny lake as a car wash.
Lake Waiporohita on the Karikari Peninsula is one of just 12 rare dune lakes in the north with outstanding ecological ratings.
But Ngāti Kahu, the Department of Conservation (DOC) and the Northland Regional Council say it is at growing risk from motorists who use it to wash down their vehicles.
The 6.9-hectare lake is on the left of Inland Road, just before the Rangiputa intersection.
Ngāti Kahu rūnanga chief executive Anahera Herbert-Graves said dune lakes in the Far North were already under multiple stresses, including nutrient run-off, invasive pest fish and water weeds.
But Lake Waiporohita faced an added threat, being one of the few dune lakes easily accessible to the public.
"Unfortunately that means this lake is all too often used as an unofficial car wash by passing 4WD vehicles that have used nearby beaches or boat ramps," Ms Herbert-Graves said.
The practice disturbed reed beds and native plants in the lake bed around the margins, with stirred-up sediment also adding to the risk of algal blooms.
The regional council's Kaitaia manager, Peter Wiessing, said car washers were also exposing the "gem-like" lake to oil and other lubricants.
"Vehicle owners should use public car washes, or wash down their vehicles and trailers at home," he said.
The council has approved $52,000 funding to improve water quality in three Far North dune lakes, and plant along the edges.
Ms Herbert-Graves said the next step would be to landscape the strip where the car washers gained entry, and put up signs warning them off.
There were also plans to manage the large flocks of Canada geese that fouled the lake.