A 40 percent greenhouse gas emissions reduction target by 2020 is achievable at little or no cost if efficiencies are widely applied , the Green Party says.

The party today released results of its own study about how to achieve such a reduction in comparison with 1990 emissions levels, and took a swipe at National over its own analysis efforts.

Co-leaders Russel Norman and Jeanette Fitzsimons detailed options for creating efficiencies in the agricultural, energy, forestry and transport sectors.

The Government will next week take an emissions target to a United Nations climate change negotiations meeting in Bonn, Germany.

Climate Change Minister Nick Smith hasn't released that figure yet, but has said a 40 percent reduction on 1990s levels -- which international scientists say is necessary to keep global warming to around 2degC -- would be prohibitively expensive.

National campaigned last year on a 50 percent cut by 2050, which would equate to about 15 percent by 2020.

The Greens today floated de-stocking in the dairy sector, along with genetically improving herds toward less emission-prone cows.

Ms Fitzsimons said when it came to dairy farming, the focus was on volume of production over efficiencies.

Overall emissions were higher per head when cows were intensively stocked, and cutting from 2.83 head per hectare to 2.43 would make a significant difference in efficiencies.

"It should make farmers happy, but Fonterra not so happy," she said.

Retiring Huntly's coal-burning power station and restricting output at Taranaki's gas-fired station, while upping geothermal and wind-generated energy production, were identified as being huge emissions savers.

Introducing staggered fuel economy standards in vehicles and promoting an aggressive forest planting regime were also part of the overall plan.

Ms Fitzsimons said the Greens had worked hard on the study over the past couple of weeks when it became clear the National Party had apparently not moved to do one itself.

But Dr Smith said in Parliament that significant work had been done on mitigation options.

"I can assure the member the Government is getting as wide and accurate advice as possible."

Dr Smith said when it came to radically altering the likes of energy production and agricultural practices, the economic costs were too extreme.

He said de-stocking in the dairy industry could cost up to 50,000 jobs and about $6 billion in earnings.

Alterations to the primary energy production sector would also cost billions of dollars, setting consumers up for huge increases in power prices.

When it came to large-scale planting, Dr Smith said foresters were aware they would get credits at planting time, but there was also nervousness about the debits they could expect to be lumped with at harvest time.

The scale of change mooted by the Greens, and their emissions targets, were not realistic, he said.

"It's going to be a major challenge for New Zealand to even get back to 1990 levels."

Ms Fitzsimons said while the Greens had left their run late, the party would pressure the Government to take a 30 percent target to the Bonn meeting, with flexibility to up it to 40 percent if other countries also agreed.

Dr Smith this morning told reporters the Government was likely to take a target range to the Bonn meeting, which could change when other countries indicated how far they were prepared to go.

It would be "incorrect" to say the Government had already made its final decision on what that would be.