Biosecurity officials in Northland are working to clear a large infestation of banned koi carp from two Kaipara farm ponds.
Northland Regional Council senior biosecurity manager Don McKenzie says more than 100 of the pest fish, some more than 50cms long, have so far been removed from the large dairy farm near Paparoa and destroyed.
How the fish came to be in the two large farm ponds is still a mystery, but inquiries indicate they have been there for several years and before the current owner bought the farm.
Koi carp are classed as a noxious pest in this country because they prey on other fish and damage freshwater plant and fish habitats.
They are thought to have been accidentally imported into New Zealand more than 40 years ago as part of a goldfish consignment.
Mr McKenzie says it's important not to confuse koi carp with grass carp which are used for aquatic weed control and are not a pest.