"Fragments have been found in several places around the lake, including some within the lake's weed cordon ... one in front of the Steep St reserve and many small plants and fragments in a northwestern bay between Boyes beach and the Department of Conservation camp."
No fragments had washed up on any beaches so far, indicating the main infestation was likely to be within the northwestern bay.
Once the full extent of the incursion was known, a response plan would be developed, Mr Corbett said.
"It is likely that hornwort may have been present in the lake for five to seven years, so it was not stopped by the weed cordon at Acacia Bay which was established more recently."
The discovery was a reminder to boaties to always inspect trailers, engine wells, anchors, propellers, fishing gear and other equipment to ensure they weren't carrying weed fragments or pest fish species between waterways.
All equipment should be thoroughly cleaned before being used again.
Meanwhile, the public is invited to a Water Quality Field Day at Lake Okaro on Friday, May 25.
Lake Okaro is one of Rotorua's most polluted lakes, with algal blooms every summer limiting recreational use.
The Okaro Catchment Lake Restoration Group, formed in 2009, works with the community and the regional council to improve the lake's water quality.
The field day will update the public about plans and projects in store for the lake.
The field day is being held on May 25 from 10.30am. Those wanting to go the field day should email meganbirchall@hotmail.com.
