The prime minister has dismissed claims from his deputy Barnaby Joyce that the Nationals secured the exclusion of livestock methane emissions in negotiations over the government’s net zero 2050 target.

On Thursday, the Nationals leader said he had successfully negotiated a “carve out” commitment from Scott Morrison to exclude agricultural methane emissions from Australia’s emission reduction task.

A report published in The Australian on Thursday indicated this related to Australia opting out of the global methane pledge, which is a commitment to reduce methane emissions 30% by 2030 that will be launched at the Cop26 climate summit in Glasgow next week.

Joyce said the document signed off by Morrison to secure the Nationals’ support for the net zero pledge by 2050 had excluded methane cuts, “100%”.

“It was absolutely within our document that methane is to be excluded – absolutely 100%,” Joyce said.

“It was diligent work that brought about a substantial process that gave us the insurance that protects regional industry, which underpins regional towns.”

Despite the meat and livestock association having a carbon neutral by 2030 target which suggests herd numbers can be maintained, Joyce said the only way to reduce methane by 30% by 2030 would be “to go grab a rifle (and) go out and start shooting your cattle.”

When asked if agriculture had been “carved out of net zero”, Joyce said the Nationals had made clear that “no deal would go forward that we would support unless it was absolutely categorically ruled out and we got that.”

It is understood that the document explicitly ruled out the methane pledge as a result of the negotiations.

Morrison, who has refused to make public the terms of the agreement struck with the junior Coalition partner, said there was no carve out for agriculture, and also confirmed the government’s position on methane emissions had not been influenced by the negotiations with Joyce.

Bill Hare, the chief executive of research firm Climate Analytics, said claims by the emissions reduction minister, Angus Taylor, that the beef industry would end if the government signed up to the global methane pledge were “completely false” and “a smokescreen covering up for inaction by the fossil fuel industry”.

He said government data showed the agricultural industry had already cut methane emissions by 16% since 2005, and official projections released this year showed it was expected to continue to fall by about 1.2% a year.

“Australia could easily achieve a 30% reduction in methane by 2030 by slightly accelerating the already evident improvements in the agricultural industry and the land sector, and by halving emissions from coal mining and natural gas production, and the waste sector,” he said.

In September, Morrison participated in a meeting of major economies at which the methane pledge was discussed, but Australia did not commit to the program at this time.

“We didn’t have any plans to sign up to that,” Morrison said, when asked if the negotiations with Joyce had changed the government’s position.

“It’s a whole of economy emissions reduction target and what we’ve said very clearly is that we are also not signing up to the 2030 methane request.

“Under our plan, we won’t be putting any mandates on farmers, we won’t be targeting them in any way as part of our emissions reduction plan,” he said.

Morrison said he did not want agriculture to be excluded from “the revenue streams and the income sources” associated with the net zero commitment, saying this would be a “great disadvantage”.

After facing a barrage of criticism for the government’s 2050 emissions reduction plan, including its lack of ambition for a revised 2030 target, Morrison acknowledged there had been a lot of “feedback”, which did not express the same “confidence in technology” as the government.

In Parliament on Thursday, Labor again targeted the government over its failure to release modelling for the 2050 pledge, seizing on evidence from the Department of Industry in Senate estimates that suggested it was being reworked.

Jo Evans, deputy secretary of the Department of Industry, Science, Energy and Resources, told senators the modelling had been “complete for several weeks” prior to cabinet’s approval of the plan on Monday evening.

However, she also said details were still being worked on hours after cabinet’s sign-off, and echoed prime minister Scott Morrison’s comments that it would still be “a few weeks” before the modelling would be made public.

While it was “absolutely finalised”, Evans said it first had to be made “accessible” for the public to understand.

In response to a question from Queensland Labor senator Murray Watt, who asked about the modelling’s current form, Evans said it included: “spreadsheets and finance and technical reports that are designed for an audience that is more sophisticated in terms of how it will understand results.”

“It is a complex set of work and we’re just finalising it to make sure that we’ve got that accurate for public consumption,” Evans said.

The Labor leader Anthony Albanese said the remarks showed that Morrison had been “misleading the Parliament and the Australian people by not telling them that the modelling document for his net-zero policy does not exist.”

Morrison said the modelling would be released some time over the next couple of weeks.

The prime minister also responded to critics who had wanted Australia to pursue a more ambitious 2030 target, saying this could divert resources away from the technology needed to achieve the longer-term cut in emissions.

The prime minister, who left Australia for the G20 leaders summit on Thursday, was also asked to give more details about the infrastructure spending promised to the Nationals as part of the net zero deal. Morrison said any new funding commitments would be announced before the next election.

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Joyce said Morrison had promised a range of other projects, including a new fund to help support regional Australia and a broader remit for the regional investment corporation.

As acting prime minister, Joyce is expected to announce further regional projects over the coming week, including on Friday when he takes part in a “bush summit” being sponsored by the Daily Telegraph.

“You are seeing now as this process goes forward the reason why the Nats had to go into bat. We hear the rhetoric at times that it was somehow a week spent doing little but it was actually a week carving out a very precise agreement,” Joyce said.

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Photograph: Tim Robberts/Stone RF