Farmers' hopes of getting an easy ride in climate change legislation has been dented by the combative stand on methane taken by Parliamentary Commissioner for the Environment.
The commissioner said to prevent global warming, methane emissions would have to fall by 10 to 22 percent below 2016 levels by 2050.
There would then need to be further reductions by 2100.
The comments came in an official report by the commissioner, Simon Upton, who is a former National Party Minister for the Environment.
The report follows intense debate about the impact of methane on climate change.
Farmers argued methane was short-lived, decaying after about 12 years, and so should not be at the forefront of climate change policy.
Federated Farmers said most methane came from livestock, and farming was essential for the economy, so it should not be burdened with climate change costs that it could not afford and did not deserve.
Instead Federated Farmers said, climate change policy should be aimed at carbon dioxide, which lasted for thousands of years.
But environmentalists argued methane's low life expectancy was not a valid mitigating factor.
Greenpeace sustainable agriculture campaigner, Gen Toop, said New Zealand must substantially reduce methane emissions from livestock.
"The explosion in cow numbers in recent decades has always been New Zealand's elephant in the room when it comes to taking action on climate change.
"The simple truth is there are already too many cows for our climate to cope with, yet the government is still allowing dairy conversions to continue - even in fragile and unique places like the Mackenzie country."
Greenpeace said the government should ban all new dairy conversations of farms.
The government has itself recognised the strength of both sides of the argument.
Its consultation on the Zero Carbon Bill offered three options, and only one of them brought all greenhouse gases equally into the greenhouse fight.
The other two options gave a softer response to methane than to carbon dioxide.
With the consultation phase over, a decision is expected later.
But the report by Simon Upton has come down heavily against methane.
"If New Zealand were to hold its livestock methane emissions constant at 2016 levels, the amount of methane in the atmosphere due to those emissions would level out within a decade," Mr Upton wrote.
"But warming from this methane would still increase for well over a century, albeit at a gradually declining rate.
"This ... is largely due to the inertia of the climate system, which is still responding to the historical increase in methane emissions from New Zealand since the 19th century.
"It would take several hundred years of constant methane emissions before warming due to those emissions ceases to increase entirely."
This view was earlier contested by several experts.
One of them, Auckland consultant Steven Cranston, argued agricultural methane should not be treated the same as carbon dioxide.
Most of it was cyclical, coming from animal fodder, and eventually returning to animal fodder, thus remaining constantly within the biosphere, he said.
By contrast, most carbon dioxide comes from fossil fuel, so it takes a one-way ticket into the atmosphere.