The government's proposed standards to make waterways swimmable are too confusing to understand, says an experienced freshwater scientist.

Photo: 123rf.com

People have until Friday to submit on the government's Clean Water package, which wants to make 90 percent of the country's lakes and rivers swimmable by 2040.

The current national bottom line was that all waterways needed to be safe for wading and boating and the package proposed to replace that bottom line with swimmable.

However, the government's plan to make those waterways swimmable had raised questions, with critics accusing the government of shifting the goalposts on the standards and effectively weakening them.

Massey University freshwater ecology professor Russell Death said it was a hard call as to whether the standards for swimmability were better than they were before.

"I've read it multiple times and I still find it just as confusing, I don't really know whether they've changed the goalposts or not [but] they've certainly confused the issue.

"For people wanting to go swimming or farmers wanting to look after their waterways, I've got a PhD in freshwater ecology, I've been working in the area for 30 years, and I still don't fully understand it."

The swimmability guidelines

The swimmable guidelines are based on the level of E coli bacteria in the water. The minimum acceptable threshold for swimming is an E coli level of 540 per 100 millilitres.

Under the package the standards take into account what the annual median concentration of E coli in a waterway is and how often the 540 per 100 ml threshold is exceeded.

Photo: RNZ / Ian Telfer

University of Otago freshwater scientist Marc Schallenberg said focusing on more than just a cut-off concentration of E.coli could be a good approach because the bacteria varies with the rate water flows.

He believed the new standards probably reflected the real world situation more.

However, he said the government had not demonstrated its claim that the new standards were tougher than the old ones, or that they aligned with the way European countries and the US assessed risk and suitability for swimming.

Dr Schallenberg said the only way to show the new standards were in line with the old guidelines, and the overseas standard, was to compare which of New Zealand's rivers would meet the swimmability standard under each set of guidelines.

He said this was raised with Environment Minister Nick Smith after the clean package was announced and Dr Smith said his people would do that analysis.

But Dr Schallenberg said the analysis had not come out yet and it was an important test.

"To me that's the key bit of information that really demonstrates that the goalposts haven't shifted, so there's a lot of talk from the ministry that they hadn't shifted, but there's no demonstration of that, so ... that is a let-down."

Not enough on the health of waterways

Dr Death said the package as a whole was disappointing, as he did not believe it had changed much from the original National Policy Statement on freshwater released in 2014, which the package was supposed to strengthen.

He said that while it was good there was a focus on E coli levels, it was just one aspect of the health of waterways with respect to whether you could swim safely.

His bigger concern was that he believed the package did not address any of the things that needed to be managed - such as nitrogen and phosphorus, sediment, flow patterns and habitats - in order to look after the ecosystem health of the country's waterways.