Bay of Plenty Regional Council is warning people to stay clear of large lumps of algal scum on the shoreline of Lake Rotorua and to keep their pets away also.
Tests so far show the algae is not poisonous, but the council says it could still be carrying toxic particles.
Environmental scientist Matt Bloxham says it's unusual to see algal blooms persist into winter.
He says the algae is teal blue, has the consistency of cream cheese, and has an irridescent ooze seeping from it.
Lake Rotorua has had problems with algal blooms on and off since 1997, because of increasing nutrient levels in the lake.