Horizons' natural resources manager Jon Roygard had explained that lakeweed harvesting was essential to prevent the lake becoming toxic, and freshwater manager Logan Brown outlined the science behind it.
The matter goes back to 2011-2012, when national science body Niwa determined that weed harvesting was needed.
In 2014 Horizons made the Lake Horowhenua Accord Action Plan with local iwi.
A weed harvesting machine was bought then, for $1.8 million.
"I said I can't see that machine doing a complete harvest by December 30, 2021. If you can do it I will personally donate $5000 to the Horowhenua clean-up fund," Cotton said.
He's now hoping he will have to pay up, but it doesn't look likely with the harvester in storage under security.
Cotton said he supported the weed harvest, but was against going into projects without definite numbers.
"We have spent over $3 million so far. At this stage the weed harvester hasn't touched the water."
Other costs have been over $500,000 for the various permissions needed for the harvest, and the cost of building a lake access road and boat ramp.
Along the way a small group of people have taken the council to court at least four times - losing each time but incurring legal costs to council.
In a separate incident, a Horizons staff member working on the project was assaulted late last year. The project has "put Horizons staff through the wringer", councillor Nicola Patrick said.
New extras are the cost of complying with consent conditions and $72,000 for security to protect Horizons' staff and equipment.
There will also be the cost of disposing of the weed, which will be dried, composted and may be suitable for addition to market garden soils.
Under the lake accord, there is also the cost of the iwi members who are Tāngata Tiaki, partners in the project and providing knowledge and security.
A lot of information is exchanged between the parties, and the partnership is an investment for the future, Horizons' chief executive Michael McCartney said.
"It's not money going out and nothing coming back."
Councillors' agreement means a trial weed harvest of 40ha will take place in the 2021-22 financial year.
Where the extra money will come from was not decided, but councillors Cotton, Bruce Gordon and John Turkington said it should only be a cost for Horowhenua ratepayers.
Councillor Fiona Gordon said she would like such costs to be more transparent in future.
McCartney wasn't part of the vote, but expressed his agreement with councillors' decision.
"We have a once in a generation opportunity to focus on the lake and make a substantive difference.
"There have been past opportunities, and people have walked away. This time we haven't," he said.
