The Otago Regional Council (ORC) is working with the Queenstown Lakes District Council, Land Information New Zealand and commercial boat operators to control the exotic weed at a site about 500m downstream from the top of the river at the Kawarau Falls Bridge.
ORC environmental monitoring and operations director Jeff Donaldson said the lake was clear of lagarosiphon ''and we're going to continue to keep it that way if we can''.
The method of control was still being considered, but he expected the operation to start in late October.
''It's been in the planning for a number of years. We've been monitoring the site because it has been expanding quite considerably over the last few years.
The aim was to ''knock back'' the weed enough to prevent it reaching the surface and getting picked up by passing boats.
''Last summer there were surface-bearing plants because the Kawarau got so low, and we don't want to risk that happening this summer.''
Commercial jet-boat operators already followed guidelines for preventing the spread of the weed into Lake Wakatipu's Frankton Arm, but monitoring private boat operators was a ''struggle''.
The other threat came from boats coming out of heavily infested Lake Dunstan and being launched at Frankton Marina.
Mr Donaldson said lagarosiphon had been an issue in the Kawarau River and Frankton Arm for about a decade.
A few plants found in Frankton Arm in 2009 were removed by hand, while another ''clump'' found near Frankton Marina last year had also been removed.
The weed was present throughout the Kawarau River, particularly in shallow areas, with a weaker current, that received a lot of sunlight.
Inspections of Frankton Arm were carried out every year in late summer, focusing on the lakeshore between the head of the river and the Hilton hotel on one side and Frankton Marina on the other side.
