John Wilson and Andrew Burton say ''hundreds'' of people are affected by lagarosiphon in the lake, and it had drawn a lot of comment from the community.
Over the Christmas holidays, children were having problems swimming, and there had been a large number of irate boat users.
The weed clogged boat ramps around the lake and created a stench, Mr Wilson said.
''What is happening now in the Bendigo [area of the lake] where the worst case of weed is, it is rafting.
"Rafts break off and float down and then end up on the shore and rot ... soon Cromwell will become the town to avoid. We can't afford to lose holiday-makers.''
The pair had taken concerns to politicians and those in charge of controlling the weed.
Boffa Miskell has been carrying out lagarosiphon control in Lake Dunstan and Lake Wanaka for Land Information New Zealand (Linz) for about the past five years. Linz is responsible for the Lake Dunstan bed.
In an email to Mr Wilson last year, a Linz employee said ''total control of lagarosiphon is the vision for Lake Wanaka."
"However, in Lake Dunstan our goal is to reduce the impact of aquatic weed on lake users and reduce the risk of users transmitting weed to other lakes in the region''.
The Cromwell men contacted National MP Jacqui Dean, who contacted Minister for Land Information, Louise Upston, who in early December said ''weed control at Lake Dunstan is a priority''.
Work was scheduled before Christmas, she said in emails. Control work was conducted less than two weeks later.
Linz acting group manager, crown property, John Hook said aerial control of weed was carried out on December 15 at a number of ''high value areas'' (HVAs) - parts of the lake commonly used for recreational purposes.
Mr Hook said the current programme had been proved sufficient to control lagarosiphon at those areas, but Mr Wilson and Mr Burton said the weed was ''way out of control''.
It was so thick in some places ''you could almost walk on it''.
When the Otago Daily Times visited boat ramps on Lake Dunstan near Old Cromwell last week, lagarosiphon lined the lake edges.
Mr Hook said some years, water conditions and the state of the weed could limit the level of control which could be achieved at different sites.
Lagarosiphon could grow up to a metre over summer, he said.
This summer's growth had been as expected, however, increased growth in spring meant continued control was required.
The Otago regional pest management strategy set control of aquatic weed at the lake's HVAs as a priority.
Mr Wilson said when working for an agrichemical company in the 1970s, he warned that if an efficient spray programme was not implemented when Lake Dunstan was filled, the weed would get out of hand.
The two men say an alternative for controlling the weed was the herbicide Diquat.
It worked well, had no negative side effects and a trial of Diquat on lagarosiphon in Lake Wanaka's Glendhu Bay in the 1970s ''virtually wiped it out''.
Otago Fish and Game Council chief executive Niall Watson said the lakeweed was a ''mixed blessing''.
He could understand why it was cleared from ski lanes and swimming spots.
Mr Watson said lakeweed was also a key component of the lake ecosystem, because it provided good fish habitat.
''While it can be a pain, even for anglers, it's also what makes this lake fishery work,'' he said.
Mr Wilson and Mr Burton said they were not on a witch hunt.
''The objective ... is to improve things. We don't want to have a go at anyone, we just want better control.''
