A conservation trust manager angry at Environment Minister Nick Smith's swimmable river stance has challenged him to a boxing match - with the loser to "frolic" in one of Christchurch's most polluted swimming holes.

Greg Byrnes posted a classified advert on Wednesday challenging Dr Smith to a "Queensbury Rules fist fight".

He asked Dr Smith to meet him at the Otukaikino Swimming Hole, which he said was one of Christchurch's most polluted waterways, but was still officially called a swimming hole.

"The loser to frolic in the water hole for no less than 5 minutes. This is in line with my target to make 90% of all members of the NZ Parliament believable by 2020," he said in the advert.

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It was sparked by Dr Smith's new swimmable rivers policy, which aims to have 90 per cent of lakes and rivers swimmable by 2040 - but has lower standards for what is considered swimmable.

Mr Byrnes is general manager of the Te Kōhaka o Tūhaitara conservation trust and a former Environment Canterbury planner, and said he saw the effect of pollution on Christchurch's waterways every day.

Earlier this month Dr Smith took a dip in the Manawatu River, in front of gathered reporters, after the river was labelled the worst in the western world and an accord was signed to improve it. He was challenged to swim in the river by Rangitaane kaumatua Wiremu Kingi Te Awe Awe, who said he was looking forward to seeing the MP in speedos.

He did not expect Dr Smith to respond to his advert, but posted it because he believed the Government policy treated people "like idiots".

"Most people will hear that and think they're doing something, and they're not," he said.

"We've got a fantastic country, but we're fast-tracking it to not a nice place. I can't imagine what the Canterbury plains will be like in 15 years, unless we do something."

Forest & Bird has criticised the swimmable rivers policy, saying it did not apply to 90 per cent of waterways, and reduced swimmability standards.

A spokeswoman for Dr Smith said he would not be responding to the challenge.

But in a written statement Dr Smith defended the standards, saying they would require 1000km of waterways to be improved each year.

"This Government is doing more to better manage water than any has previously done," he said.