Drivers are washing their cars in a once-pristine freshwater lake in the Far North after visiting nearby beaches.

As a result, the environmentally fragile dune lake near Tokerau Beach on the Karikari Peninsula has been exposed to oil and lubricants as well as fish and weed pests.

The lake has been cursed by being too open to vehicles and being mostly shallow, with a maximum depth near the centre of just 3.5m.

Te Runanga-a-Iwi o Ngati Kahu chief executive Anahera Herbert-Graves said dune lakes were under multiple stresses, including nutrient runoff, invasive pest fish and water weeds, and cumulative effects from use by the wider public.

However, Lake Waiporohita has an added threat in being one of only a few easily accessible dune lakes in Northland.

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"Unfortunately, this means that the lake is all-too-often used as an unofficial car wash by passing 4WD vehicles that have used nearby beaches or boat-ramps."

Last September, the Northland Regional Council (NRC) agreed to spend $52,850 to improve the lake's water quality and ecology, as well as Lakes Rotokawau east and west on the north-western corner of the peninsula.

The project will include fencing, exotic tree removal and riparian replanting to allow the lake to filter land-based nutrients.

Herbert-Graves said the next phase would involve landscaping the waterside strip in the open area along Inland Rd where four-wheel drives tend to access the lake

"This area will be landscaped shortly using locally sourced materials, native plantings and signage to let the public know that vehicle and boat access will no longer be tolerated in the lake."

Peter Wiessing, NRC Kaitaia area manager, said vehicles getting in the lake increased the risk of introducing water weeds and pest fish, as well as exposing it to oil and other lubricants.

One of only 12 dune lakes in Northland with outstanding ecological status, the 6.9ha lake's riparian zone will be planted, along with other work to protect the immediate environment.

The Department of Conservation, the manager of the lake-bed and marginal strip, and Landcorp, which farms the Rangiputa Station surrounding the lake, are also involved in the protection project.

The partners are also are keen to work with Fish and Game to better manage large numbers of Canada geese that gather at the lake and foul it with their waste.