The Nationals have called for agriculture to be exempt from a net-zero target - a move which now appears at odds with the National Farmers Federation (NFF), which says agriculture is "too important to leave out".

Nationals senator and former resources minister Matt Canavan has declared he would be prepared to "cross the floor" if the Government moved to legislate to go carbon neutral by mid-century.

His backbench colleague Barnaby Joyce has raised similar concerns.

There is no certainty that a bill would be required, but the incendiary remarks show that some members of the Nationals now stand in open defiance of the Prime Minister after a week of tacit approval.

Deputy Prime Minister and Nationals leader Michael McCormack said an exemption for agriculture in such a target "may well have to be on the table".

He's also pointed to other industries in regional areas, including mining and manufacturing, that should be considered for special treatment to ensure regional communities are not worse off.

"They want to be able to have jobs and whether they are in mines, whether they are on farms or in factories they don't want that factory or that farm to have to close to meet some goal that's put in place to achieve an outcome in 2050."

That's not the way the nation's peak farming body sees it.

NFF chief executive Tony Mahar said the group's clear position was to support reaching net zero by 2050, while ensuring "agriculture is not worse off".

"Agriculture is in a unique position — different to any other industry in that farmers can sequester carbon and reduce emissions. Agriculture is too important to leave out and too important to ignore," he said.

"Farmers are in the box seat to seize the opportunities from a reduced emissions future — and many are already doing just that. Any policy that restricts opportunities available to farmers and rural and regional communities would clearly be a negative outcome."

The NFF has argued that more work is needed to understand how the industry can help drive down emissions before the industry's role in any national emissions target is determined.

"Care needs to be taken that agricultural land does not get transferred into carbon sinks that are subeconomic, havens for feral plants and animals and a fire risk.

"Offsetting is a legitimate solution that must meet economic viability thresholds that allow benchmarked income and proper management."

Mr McCormack said he had been consulted by the Prime Minister ahead of last week's speech and was comfortable with assurances that regional communities were being taken into consideration.

While some within his Nationals party room are pushing for the exemptions as a compromise, Senator Canavan said he was not prepared to accept an amended policy.

"I'm opposed to it holus-bolus because it would shut down coal mines, it'll shut down factories," he told ABC News.

Tony Wood, energy program director at the Grattan Institute, says any carve-out for agriculture would be a "home goal" because it would make achieving the net-zero target harder and more expensive.

"It's not in the interest of the agricultural sector because many participants in the agricultural sector are already contributing to lower emissions and would like to see, I'm sure, the carrots and sticks to help them keep going," Mr Wood said.

"And it's certainly not in the interests of the other sectors of the economy such as transport and heavy industry that would have to do more."

Story By | Anna Henderson and David Lipson