The flourishing weed in Lake Dunstan is a safety issue, the lake harbourmaster says.
Ongoing problems with controlling the lagarosiphon weed were discussed by Central Otago district councillors this week.
Lake Dunstan enforcement-education officer Shayne Hitchcock highlighted the issue after presenting his report on lake activity over summer.
He said the weed spraying done in December was ineffective and he fielded complaints about the weed throughout the summer holidays.
''It's a safety thing as far as I'm concerned. Small kids are getting tangled up in the weed.''
Mr Hitchcock said eradicating the weed was impossible but more attempts should be made to control it at high-use recreational spots around the lake, as safety was important.
The growth of lake weed at Champagne Gully had been so extensive the boating area had been reduced.
That was causing congestion, which was also a safety hazard, he said.
Boffa Miskell Ltd carried out lake weed control for Land Information New Zealand (Linz) and biosecurity project manager Marcus Girvan told the council earlier in the meeting the lagarasiphon was sprayed with herbicide on December 15.
He thought there was a good result at most sites.
Nine sites were treated and 16 sites were targeted for further aerial and boat-based spraying in May or June, depending on the weather, water and weed conditions.
Lake weed in Bannockburn inlet had not been sprayed before Christmas as there was an algae in the inlet which would have made the spray ineffective, he said.
Cr Shirley Calvert, of Cromwell, said kayakers, boat users, swimmers and waterskiers were all affected by the amount of lake weed.
''From a lake user's point of view, the lake was barely usable [over summer], especially for kayakers.''
Mr Girvan said lowering the lake level to expose the weedmats had been mooted as another control option but that probably would not be particularly effective.
It might ''knock it back'' but would not kill it.
The timing of weed control using herbicide took into account several factors.
It had to be timed outside peak recreational use of the lake and after consultation with orchardists and grape growers, who drew water from the lake.
Control methods had to be as cost-effective as possible as there was a limited budget for the work, he said.
Central Otago Mayor Tony Lepper said the weed was difficult to control but Cr Stu Duncan said it was no different from trying to control the spread of land-based weeds like gorse or broom.
Andrew Burton and John ''RJ'' Wilson, of Cromwell, aired their concerns earlier this year about the volume of weed in the lake and said hundreds of recreational users were affected.
