Two aquatic weeds have been found for the first time in Rotorua's Lake Rotoma.
The weeds, hornwort and egeria densa, were found inside the Bay of Plenty Regional Council's weed cordon by summer students who were working for the regional council.
The students were warning boaties about the dangers of not checking and cleaning their boats before they entered the water.
Regional Council general manager of natural resource operations Warwick Murray said he was disappointed the weeds had turned up in the lake but was pleased the weed cordon was working to keep aquatic weeds out of the main body of the lake.
Weed cordons that trap weed fragments have been installed on four lakes. Rotoma and Okataina have two each, and Okreka and Rotoehu each have one cordon.
They are a world first and Mr Murray said they were successfully preventing weeds spreading into lakes from boats and equipment.
"It's a timely reminder of how important it is to clean, check and dry boats, trailers and anchor wells before you leave a lake to make sure you don't carry unwanted hitchhikers into another lake. There are signs up at all Rotoma boat ramps reminding boaties to check, clean and dry their equipment, not just let the weed cordon do the job for them," Mr Murray said.
"Rotoma is a very beautiful lake and has been free of these weeds that are already in nearby lakes. Lake Rotoehu has hornwort in abundance. The weed cordons are a stopgap against laziness, ignorance and oversight, but it is the responsibility of all boat owners and users to ensure these lakes remain weed-free."
Mr Murray said the regional council had done extensive checks and had not found any sign of the two weeds in the rest of the lake so he was hopeful the cordon had done its job.
The lake would be monitored for the next five weeks to ensure the weed had not established elsewhere.
He said hornwort and egeria densa were 'containment pests' - weeds which could not be eradicated from the region but needed to be contained to limit their impact on other lakes.
Mr Murray said they were banned from sale, propagation and distribution throughout New Zealand.
"The two plants are very distinctive and different from the native species lagarosiphon major, which is already in Lake Rotoma."
In March 2010 hornwort was found in Lake Okataina, and was suspected to be in Lake Okreka last summer, but due to an infestation of egeria densa, the extent of the spread could not be measured.
Mr Murray said 85 per cent of people interviewed last summer around the Rotorua lakes had a high level of interest in aquatic pest issues, and 45 per cent checked and cleaned their boats before moving to another lake.
At Lake Rotoma this summer, 51 per cent of people interviewed were unaware or had a low awareness of aquatic weed issues, but 49 per cent said they had a moderate or high awareness.