The government's decision to set a target for methane reduction is not based on science, the National Party says.
In its Zero Carbon Bill, the government proposes that biological methane emissions should be reduced by 10 percent by 2030, and - provisionally - by between 24 and 47 percent by 2050.
National Party climate change spokesperson Todd Muller said setting the latter target went against the advice of the Climate Change Chief Executives' Board and the Parliamentary Commissioner for the Environment.
"This target around where we want to anchor the trajectory of this country over the next 20 years. . . we have to make sure that's underpinned by science.
"The National Party strongly believes that what's been landed certainly doesn't deliver against that."
However, Climate Change Minister James Shaw has defended the longer-term target, saying it was based on the Intergovernmental Panel on Climate Change's scenarios for living within 1.5 degrees of global warming.
"That's where a lot of the confusion has come from. So, Dave Frame and the Parliamentary Commissioner for the Environment. . . the reports were solid but they were based on different assumptions.
"And that's why we didn't use them because they didn't actually meet that requirement of living within 1.5 degrees."
Mr Shaw said a nominal level had to be set to give the agricultural sector and the wider economy some certainty.
The Zero Carbon Bill requires the Climate Change Commission to report back by 2024 with a specific number.