A waterway that Environment Minister Dr Nick Smith once deemed "impossible" to improve to a swimmable condition would be considered swimmable under proposed new standards.

Several other waterways – including dry shingle beds and rivers with toxic algal blooms – would also meet the swimmable standard, despite clearly being unsuitable for swimming.

Critics have said such instances reveal shortcomings in the way the proposed standards are measured.

TETSURO MITOMO/FAIRFAX NZ Washdyke Lagoon near Timaru in 2017.

Smith said because the policy applied to such a large number of rivers and lakes, there would inevitably be "exceptional circumstances."

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During a speech about freshwater management at Lincoln University last year, Smith explained why the government would not require all waterways to be swimmable.

JOHN BISSET/FAIRFAX NZ Washdyke Lagoon photographed in 2012.

Citing Washdyke Lagoon near Timaru as an example, he said some places would likely never meet swimming standards due to natural events.

"Water bodies like the Washdyke Lagoon in Canterbury... are home to many birds whose E.coli make it impossible to meet the swimming standard without a massive bird cull," he said.

But under the government's proposed standards, Washdyke Lagoon has a "fair" grading for swimming, which is considered swimmable.

LAWRENCE SMITH/FAIRFAX NZ Prime Minister Bill English and Environment Minister Dr Nick Smith announce a plan to clean up New Zealand's waterways.

The lagoon's median E.coli levels are nearly three times the safe level for human contact, 1400/100mls. At times they reach 2900/100mls, nearly six times the safe level.

Under the standards, swimming quality in lakes is determined by cyanobacteria concentrations (toxic algae); for rivers, it is determined by E.coli concentration.

Washdyke Lagoon passes the standards because it has safe cyanobacteria concentrations, despite high levels of E.coli making it dangerous for swimming.

LAWA/SUPPLIED The ephemeral North Ashburton river is dry for long periods, but would be graded as having "good" swimming quality under proposed new standards.

Smith said that of the 800 lakes measured under the new standards, Washdyke Lagoon was the only one that fit that category.

"Washdyke Lagoon is the only one in the unusual position of having a toxic algae level described as fair but an E.coli level that makes it unsuitable for swimming," he said in an interview.

"Whenever you try and introduce a national grading system over 45,000 kilometres of river and over 800 lakes, and each of them have different characteristics, you do get some exceptional circumstances and Washdyke Lagoon is in that category."

SUPPLIED The Taylor River in Blenheim has "very poor" quality under suitability for contact recreation guidelines, but is swimmable under proposed new standards.

He said it was marked on the ministry's website that the lagoon had high E.coli levels.

Other examples of swimmable rivers not suited for swimming include rivers which partly dry up, including the Ashburton and Selwyn rivers in Canterbury.

'A RELATIVELY NEW PROBLEM'

In some cases, rivers with low E.coli concentrations are deemed swimmable, even if they have high cyanobacteria levels.

Toxic algae has been a growing problem in rivers and lakes around the country and is a particular issue in rivers with otherwise good water quality.

Over the last 10 years, rivers known to have toxic algal blooms increased from fewer than 10 to more than 100, said Cawthron Institute senior scientist Dr Susie Wood.

"Benthic blooms in rivers are a relatively new problem," she said.

"Our research indicates that increased concentrations of nitrogen and fine sediment promote these blooms, both factors that intensify when land use is modified.

"Blooms are also more likely to occur in summer, when there are long periods without rainfall which wash the cyanobacteria out of the rivers."

Rivers such as the Opihi near Timaru and the Hurunui in North Canterbury had high swimming water quality, but toxic algal blooms meant they were unswimmable for some of the summer months.

Wood said it was pleasing that the government had committed to improving waterways, but a lack of data provided difficulties.

Less than 5 per cent of the country's lakes had reliable cyanobacteria concentration data, including only three South Island lakes, she said.

It meant swimmability standards for lakes were reliant on modelled data, with predictions based on "limited and biased" datasets.

During a peer review of the new standards by the Cawthorn Institute, the decision not to account for E.coli levels in lakes was described as a "major omission."

Smith said the standards focussed on the primary risk to human health in each waterway type.

"The greatest risk for human health in the 45,000 kilometres of rivers is E.coli, where for the 9000 kilometres of lakes, the risk is toxic algae.

"There are a small number of examples where you have a river where toxic algae is the issue and a small number of lakes where E.coli can be the issue."

He said forcing councils to measure variables less likely to present a health risk would impose undue pressure.

"The difficulty with that is that we're forcing on councils, as part of the policy, to have to measure these things.

"To put a compulsory requirement to measure E.coli in 800 lakes when it's only a problem in three... was the reason that the model works this way."

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