People opposed to a large irrigation scheme in Mid-Canterbury want their side heard when consents are considered, and say nitrate pollution is a concern.
An independent commissioner is to consider the MHV water cooperative's bid for a replacement consent to irrigate 56,000 hectares of private farmland for the next 10 years.
However, opponents feared higher levels of nitrate would be discharged, which they said could affect drinking water.
The public cannot make submissions, because they were able to do so as part of the original consent process.
But environmental group Aotearoa Water Action believed their views should be taken into account when the replacement consent is considered, too, and said there was a case to challenge that.
The group hoped to garner support in a meeting at Burnside High School on Wednesday night, organised by independent freshwater advocate Angus Robson.
Robson said seven people would speak, including Otago University public health experts, a GP, an Extinction Rebellion campaigner, anglers, and a district zone expert.
MHV operates on the Canterbury Plains inland from Ashburton, and has permission to draw water from the Rangitata and Ashburton rivers in specific conditions.
Its website said it had 320km of open races, and about another 100km of piped infrastructure, and was an environmentally responsible organisation.
However, Robson said the scheme was a major polluter if the past, present and future effects of it were taken into account.
"When our health, environment and future are at stake in our own district, being able to voice our opinions and feelings is important. Renewing this consent could spoil the pure aquifers that Canterbury is famous for and the effects could remain for over a thousand years.
"It's not right that the public will not get a say in the future of their drinking water."
Nitrates were coming under increasing scrutiny for their health and environmental impacts, including a government taskforce investigation currently underway.
Robson said his calculations showed the scheme's nitrate accumulation could cause very high levels of nitrates in water.
International research indicates an association between nitrates in drinking water and bowel cancer.
More research was being done to understand what that meant for New Zealand, but bowel cancer is the second highest cause of cancer death in the country.
In February a preliminary study found up to 800,000 people were exposed to levels of nitrate in their drinking water that could be potentially harmful.
Robson said the issue was hugely important for those living in the area, and the consent renewal should be publicly notified.
"The legislation of the NPSM [National Policy Statement for Freshwater Management 2020] says if you're going to degrade a freshwater management unit - which is a catchment, such as in [MHV] Mayfield Hinds Valetta - you have to make a plan change to stop that happening, and we're saying 'you should make that plan change'."
"What the meeting's partly about is restoring democracy to Canterbury, because it was taken away when the Ecan Act came in, in order solely to enable these large scale irrigation schemes. And once the irrigation schemes were set in concrete, then Canterbury was allowed it's democracy back."
An MHV Water spokesperson Melanie Brooks said the water consent was in line with the Land and Water Regional Plan and there had been extensive community engagement about the plan.
She said MHV Water was proposing a 39 per cent reduction in nitrate leaching within 10 years, although their consent conditions have not yet been finalised.
"I think it is also important to note that we started groundwater and surface water sampling in 2016 and this has now expanded to 130 groundwater sites at our most recent quarterly round of sampling.
"We have the most extensive monitoring for our catchment and share our results with stakeholders to ensure transparency and so we are all working with the most up to date and comprehensive information. These results show that over the last 15-18 months we have an improving trend in groundwater quality."