The Modellers Pond has been plagued with weeds and algae for 20 years. Photo: Kate Russell.

Back to the drawing board for Modellers Pond


Nelson City Council is once again scrambling to find a permanent solution for the weed-plagued Modellers Pond after a $220k treatment trial failed to work.

After a lengthy debate at last Thursday’s full meeting, councillors voted eight to five to investigate the option of a full upgrade of the Tahunanui icon, which involves installing a concrete base.

It’s a solution that was originally put forward in 2017 but was cast-aside due to its $1.7 million price tag. That cost could now blow out to $2.3 million.

The pond has been beset with weeds and algae for two decades – ever since the chemical weed killer copper sulphate was banned. The weeds make it unusable for extended periods, and it requires regular manual cleaning at considerable cost.

Council have reviewed a report on the Diatomix trial carried out over summer, and an independent scientific analysis concluded there are too many variables to know whether the treatment would resolve the algae issue long-term.

Diatomix is a micronutrient solution, using nano-scale silica with ten micronutrients only bio-available to diatom algae.

The trial started in August 2018, and after three months, council approved to extend the trial for a further five months, even though the report says that “officers did not support” its continuation.

During the trial’s extension, algae continued to grow, although it was less than previous years.

Over the course of the last six years, council has spent just over $1 million on the pond, consisting of $650k operational expenditure and $350k capital expenditure.

The focus has now been put back onto other options, with the preferred option being the full upgrade, as considerable investigation work has already been undertaken on this option and it has already been granted resource consent.

It would involve installing a concrete base and improving water circulation.

However, a final decision won’t be made until early next year after the election, as council want to consult more fully with iwi on the option, or any other alternative they consider appropriate.

A report will come back to a future sport and recreation committee meeting finalising the preferred approach, which will then be put to the wider community for feedback.

Several councillors tried but failed to amend the decision to have other options explored as well, so if iwi consultation lead to another option being selected, council wasn’t put back at “square one”.

Deputy mayor Paul Matheson says, with having tried a scientific approach, it now makes sense to focus on a construction-based option, where the research and design are already well advanced.

“Although the final decision will happen at the council table, rest assured we’ll be seeking the thoughts of all Nelsonians on this option, so we have all the information needed to make a good, and lasting decision.”

Nelson Modellers Society spokesperson Nigel Gibbs told Nelson Weekly that the decision is not as “clear cut” as they had hoped.

“While this is our preferred option, what we don’t understand is why we need to go back to iwi to reconfirm the plans that were already approved.”

Funding for the project will be subject to approval via the annual plan process with the goal being to have the preferred option ready for construction from 2021 to 2023, depending on the option chosen.