Environment Minister Nick Smith is facing a stream of criticism over his plans to make the country's waterways great again.
But Dr Smith told The AM Show on Friday his opponents are using "junk science".
"This is a plan that's going to result in New Zealand waterways being more swimmable than anywhere in the world. It's a plan that's going to have them better than what they've been since World War II, or in my lifetime."
The Government wants 90 percent of New Zealand's rivers and streams "swimmable" by 2040. But to do so, it's changed what qualifies as swimmable.
'A' grade water will now be allowed to have up to 540 E. coli bacteria per 100ml, up from 260. Greenpeace says the new standard has doubled the number of swimmable waterways "with the wave of a magic wand" from 38 percent to 72, within reach of the Government's interim 2030 target of 80 percent.
Dr Smith doesn't dispute that figure, but disagrees with claims the new standards will result in a 50-fold increase in the likelihood of contracting campylobacter - the bacteria responsible for the Havelock North gastro crisis - from one-in-1000 to one-in-20.
"There's three categories of swimmable rivers - excellent, good and fair. In that lowest category of fair, more than half the time the chance is less than one in 1000."
More than half the time, but not all of the time. Using the Hutt River as an example, Dr Smith says the risk will only increase to one-in-20 about 15 percent of the time - "during a flood event, when you're highly unlikely to be swimming anyway".
"The vast majority of the time, the risk is less than one-in-1000 - i.e. it is junk science for people like the Greens to say that the Hutt River, you've got a one in 20 chance of getting sick."
Dr Smith says the new standards are "exactly the same" as those used in the US and Europe, and despite New Zealand's clean-and-green reputation, aiming for better isn't practical.
"The day the Green Party says my policy is good enough, I'm going to have a heart attack and drop dead because they're unsatisfiable."
It hasn't been just the Greens and environmental groups complaining about Dr Smith's changes however. Sharen Hansen, chair of the Rural General Practise Network, says 23 years is too long to wait for safe water.
"This Government's water policy has been compromised by desire for economic growth rather than the long-term view on environmental sustainability," she said on Thursday. "Accepting second-rate water quality standards merely continues the behaviour of short-term gains at the expense of our children's futures and New Zealand's sustainability."
"They're incorrect," Dr Smith told The AM Show.
"I'm more than happy to go to the Hutt River for a swim with me and my kids."