Wild rice is threatening to overrun farmland and waterways in Northland.
Manchurian wild rice, also known as rice grass, has colonised more than 350 hectares along the Northern Wairoa River and its tributaries and side drains in the Kaipara district.
Once it becomes established, the Northland Regional Council says it forms a dense mass which makes it one of the most difficult weeds to eradicate.
The council has begun a five-year programme to stop it spreading any further.
Manchurian wild rice is thought to have arrived in clay ballast bricks carried by ships coming to Northland for the timber trade early last century. It looks a bit like native Raupo and can grow three metres tall.
Small infestations are also established in the Auckland, Waikato and Wellington regions.