Alarmed Northland authorities are now searching for a koi carp after 10-year-old Zeke Voschezang reported seeing one of the unwanted freshwater pest fish in Kaipara's Lake Taharoa. Koi carp - a major risk for Northland's Kai Iwi Lakes Photo: Supplied / Northern Advocate The koi carp is the first seen in the 50,000-year-old lake - New Zealand's biggest dune lake and among the best examples of its type in the world. "At first I thought it was a ginormous goldfish," Voschezang from Hamilton, said of the 40-centimetre-long, bright orange fish he spotted in the lake north of Dargaville. Dune lakes are among the rarest and most threatened aquatic habitats in the world.