Following the contamination of Havelock North's water supply, which caused more than 5000 people to be affected by campylobacter last month, fingers were pointed at the Tukituki river as a possible source of the E.coli found in the Brookvale bores.
After conducting additional sampling, the Hawke's Bay Regional Council has stated it is unlikely the river was the cause of the contamination.
"The Tuki is safe to swim in!", the poster reads, "(and has nothing to do with the recent water contamination)."
"So to celebrate the effluent abundance of spring in our region and to prove we have absolutely no cause for concern, come and see your councillors put their bodies where their mouths are (and where our's drink from)."
Mr Wilson said there had been some concern the river was not swimmable, but while there were low levels of E.Coli it was well within the scale of swimmable standards.
"But it'll be too jolly cold me to be swimming [on] Sunday," he said.
Mr Yule said the poster was probably "a stunt," he said.
"Whether it's people trying to raise the profile of the Tukituki river, I don't put much weight on it. Funny things happen at election time."
Regional councillor Tom Belford, who has spoken out on the health of the Tukituki, thought the poster was "underscoring people's frustration about the situation".
Spending yesterday door knocking, Mr Belford said it was clear the issue on everybody's mind was water.
People he had spoken to "think [HBRC] and [HDC] don't have their act together when it comes to water".
Although he doubted councillors would get involved with the event, he was happy people were getting interested in the issue.
As well as a swim, the unknown creator of the poster also advertises fun games on the day, including "race to the economic bottom", and "Marco Polio".
"Go on, come see them E-cool-i off in the environment they have created," it reads.