Northland Regional Council has tested the Basin water five times this year, and twice - In March and September - it has recorded bacteria levels above that considered safe for swimming.
In March, the levels of enterococci bacteria, which indicated contamination from animal or human faeces, was more than 10 times safe level for swimming.
NRC regulatory services group manager Colin Dall said the two occasions when enterococci bacteria levels were above the guidelines followed periods of "moderate rainfall" of 15 to 38mm in the previous 24 hours.
Ms White said the optimistic event would highlight what was being done to clean up the waterways which weaved throughout Whangarei, and what work was still to be done. She said it was entirely possible to undo the damage done to waterways by farming and other industry.
There would also be a petition for people who wanted to get behind the clean-up cause.
"You can come down, and you don't have to jump in, but definitely sign that petition," she said.
There would be health and safety support for swimmers. "In terms of how hygienic it is. I don't think it will kill you. I can't imagine people will want to stay in it for too long," Ms White said.
"I'd say turn up on the day and see how you feel. We have got an option of jumping in that keep your head above the water and we're going to have a hose available."
Clean rivers campaigner Millan Ruka would be testing the water quality on the day, and would continue to do so each year, as the swim was designed to be an annual event that would track the clean-up progress.
Mayor Sheryl Mai has vowed to be the first to take the plunge. "She puts her hand up and said 'I'll be first, I'll jump first' and I'm holding her to that," Ms White said.
Whangarei district councillor Tricia Cutforth had also been involved in organising the swim and said in term of farming-related pollution "the time for dangling carrots is pretty much over".
"I'm hoping we will see a positive change with the new regional council," Ms Cutforth said. "We can no longer put it off and say we need to give farmers more time to fence."
Take the Plunge is happening at noon on November 20. Meet at the Canopy Bridge.