An aquatic pest has made its way into Lake Okareka in Rotorua.
The bright green water weed, known as Hornwort, which has not been seen in the lake before, was discovered by a boatie in April.
Bay of Plenty Regional Council's land resource manager says the biggest concern is the cost of cleaning up the weed, which appears to be spreading.
Greg Corbett says once scientists have determined how far it has spread, the council will develop a plan to remove it.
Mr Corbett says in the meantime, all boaties who use Lake Okareka should remove any weeds attached to their trailer or vessel.