The binding referendum means that if 75 per cent or more of the votes cast are in favour of the closure, the supply will be closed. If more than 25 per cent of the votes are against the closure, it will remain open.
Each property is required to have a rainwater tank, and with ratepayers set to foot at least part of the upgrades bill, permanent resident Stu Baker said residents would be "effectively paying twice".
"The wealthy . . . they're going to vote to upgrade because they can afford it. They'll vote to keep it on because it's a convenience. They rent their baches out, it's like a business for them and they want accessible water.
"The others who can't, they'll probably have to sell and move, because they won't be able to pay for the upgrade."
This was echoed by 25-year resident Lis Battes, who felt residents had not been given the information they needed before the vote.
"People have lived here for most of their lives and they don't know what's going to happen," she said.
"What it boils down to is we're going into a referendum with so much uncertainty . . . about what this is going to look like, how much its going to cost, and how sustainable it is."
She said residents had not been given figures on how much the upgrade would actually cost or how it would roll out.
"People are really worried in Mahanga, but there's so few people here, we can't do anything, all we can do is wait for our paperwork in August."
Last year it was expected that capital costs per connection for upgrade would be less than $5000 per property and operational costs would be similar to Wairoa water charges.
Wairoa's Long Term Plan 2015-2025 notes a sum of $200,000 was allocated for a treatment upgrade to the Mahanga supply before 2020.
However yesterday Wairoa mayor Craig Little said it was expected - at this stage - that the upgrade would cost "$500 to $800 a household".
If the referendum result was to keep the supply open - and thus upgrade it - residents would pay for this with a targeted rate.
When asked if there could be financial help to pay for the upgrade, Mr Little said they had not been eligible for any government subsidies for drinking water improvements - some which have deprivation index criteria.
The community's deprivation rating has been driven up by the number of holiday-home owners.
"It's been a long process, it's a very divided community on whether some want water some don't. The reason we've gone to referendum is we want the community to make the decision," he said.
"We need to get this decision made, one thing we cannot do is supply unpotable water, with what happened in Havelock North I don't want to risk anybody getting sick."
He added council would ensure all residents who were eligible to vote received all the information they needed to make an informed decision.
The supply - which originates from a shallow bore - has been designated as non-potable, and does not meet the New Zealand Drinking Water Standards (NZDWS) because there is "no effective barrier to waterborne disease entering the drinking water supply".
The supply also lacks enough pressure to be suitable for firefighting - even though there are hydrants in the reticulation.
Mr Little said if the referendum's threshold was not met, the council would make a decision on its future.
The council was not able to comment before the time of print.
*The Electoral Roll to be used for the referendum closes at 4pm on Thursday, August 3. It can be inspected during normal business hours at the Wairoa District Council Offices.
*The referendum will be held on Friday, September 29.
