New Plymouth National MP Jonathan Young has been slammed by Labour Minister David Parker after he criticised the Government's freshwater proposals. (File Photo)

The Environment Minister's office has rubbished National MP Jonathan Young's claims that the Government's freshwater proposals will cost Taranaki households more than $100,000 by 2050.

In a press release issued on December 27, the New Plymouth MP said economic modelling from a DairyNZ-commissioned report showed the proposals, focused on managing high nutrient levels to improve water quality, would cost Kiwi households $38,000 on average by 2050 and Taranaki households $120,000.

"The Government's plans will be devastating for regional employment, with a projected 1000 Taranaki households left jobless, not just from dairy farms, but many service companies that support the industry," the release said.

Young claimed the rivers that ran off Mt Taranaki did not comply with the Government's proposed targets, so no matter what farmers did there was no way of meeting them.

A Taranaki Regional Council-commissioned report also predicted a significant cost to the Taranaki region. The report, by Land Water People economist Simon Harris, said the proposals would cost Taranaki between $46m and $60m annually, which would lead to farmers becoming insolvent.

Young said freshwater was a complex issue and the Government was proposing a simplistic, one-size-fits-all approach, and a National-led Government would do things differently.

When asked for an explanation of how, he said National would take a measured, science-based approach that wouldn't hamstring the country's most profitable sector and build on work carried out in the past 25 years.

But he did not elaborate and did not provide any potential costings.

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In an emailed response, Vernon Small, the senior press secretary for Environment Minister David Parker, said Young was misinterpreting the proposed reforms.

Small also questioned the timing of the Young's statement, given it was sent out two days after Christmas and weeks after the October release of DairyNZ's report.

"The quality of freshwater was declining under National and this Government has put together proposals that will turn this around and restore our waterways to health."

Hamish McNeilly/Stuff Labour Minister David Parker has criticised New Plymouth National MP Jonathan Young for claims he made about the cost of the Government's freshwater proposal. (file photo)

Small said several commentators had questioned DairyNZ's interpretation of the economic analysis.

"But even then - and without the DairyNZ-commissioned report taking all the benefits from the proposed changes into account - that analysis shows that on the most pessimistic scenario there is a net benefit to New Zealand's economy from the proposed changes."

In his rebuke of Young's claims, Small also pointed to a memorandum from University of Auckland economist professor Tim Hazledine released by think-tank The Environmental Defence Society (EDS).

In a statement released on December 18, EDS chief executive Gary Taylor said Hazledine concluded DairyNZ had misinterpreted its economic modelling and misrepresented the impact of the freshwater reforms.

"We were concerned that DairyNZ had not accurately described what the modelling it commissioned said, and that it was putting a self-serving spin on the data," Taylor said.

"It is our contention, supported by Professor Hazledine, that DairyNZ grossly overstated the potential negative impacts of the reforms on the sector and on the wider New Zealand economy, when its own modelling shows the opposite."

Hazledine's conclusion was New Zealand's economic wellbeing would be slightly improved by the imposition of the proposed freshwater quality standards, Taylor said.

DairyNZ's principal economist and former Waikato University professor of environmental economics Graeme Doole, who prepared the report, stood by the figures and said they were arrived at using complex calculations of forecasts and predictions.