Ironically, high contamination has seen the plug pulled on a mass plunge into Whangarei's Hatea River aimed at highlighting poor water quality.

Town Basin water samples taken on Thursday returned faecal coliform levels nearly 30 times higher than those recommended as safe and enterococci (a streptococcus) more than 30 times higher.

Take the Plunge was due to happen tomorrow, following a similar event last November when people leapt off the Canopy Bridge into the river.

Tomorrow's plunge was also meant to demonstrate how Whangarei Harbour and waterways should be clean enough for swimming, but organiser Hannah White cancelled it because of the level of bacterial contamination.

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Following high rainfall and high tides, there would also be debris in the water.

''It's unsafe. We have to cancel. We can't swim,'' Ms White said.

Despite jumpers having to sign a waiver, it would be irresponsible to go ahead for health and safety reasons, she said.

''We will plan for another Take the Plunge event next summer. In the meantime there are plenty of people on the ground doing the work towards making this happen.''

People could still add to the call for more clean-water action by the Whangarei District and Northland Regional Councils (NRC) through an online petition. https://our.actionstation.org.nz/petitions/swimmable-hatea-2020?source=facebook-share-button&time=1516321221

Samples taken on Thursday from the NRC's Town Basin monitoring site, which were tested for three different types of faecal bacteria, showed the water quality was ''unswimmable''. (E.coli, 2359/100ml; Enterococci 1904/100ml; Faecal Coliform 2000/100ml.)

NRC had suggested earlier in the week no one should 'take the plunge' as it was expected there would be higher than usual levels of faecal contamination.

Historically, levels rose after a couple of days of rain, regulatory services group manager Colin Dall said.

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Run-off into the Hatea River came from upstream farmland as well as urban areas in the catchment. The NRC's and Water Safety NZ advice is to avoid swimming for two to three days after heavy rainfall in any waterways, and don't swim where water is dirty, smelly or scummy.

Whangarei mayor Sheryl Mai was one of about 40 people who braved the water in last year's Take the Plunge.

Ms Mai said she jumped to show her faith in work done by recent councils to clean up waterways, mainly through upgraded wastewater and sewerage systems.

That jump was on the same day NRC tests showed E.coli levels within the safe range, Ms Mai said.

''If the levels taken before this jump showed higher than safe levels, and I do believe that would because of run-off, I would not jump,'' she said before tomorrow's event was called off.

''Really, however we demonstrate it, we all want the outcome where we have harbours and rivers everyone can swim in and enjoy.''

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NRC tests at Whangarei swimming sites last Monday, showed Raumanga Stream was the only one with E.coli levels elevated enough for a cautionary warning on the LAWA.org.nz/swim site. Other Whangarei sites, including the upper Hatea River, were green for go at that stage.

The picture can change quickly. An early December reading at the upper Hatea River showed it in the ''unswimmable'' category, while last Monday it was deemed safe and this weekend it is flagged red for danger.

Two of the estimated 40 who took part in November's Take the Plunge reported feeling sick afterward.

They were Ash Holwell, last year's Green Party candidate for the Whangarei electorate and behind the Make the Hatea River Swimmable by 2020 campaign, and Whangarei District Councillor Trish Cutforth.