Net-zero plan 'wildly overinflated' how much carbon nation's soil can hold

So far the government has resisted calls to release the data, with Prime Minister Scott Morrison told the parliament the modelling would be released "in the next couple of weeks".

On Thursday, the Department of Industry, Science, Energy and Resources revealed in senate estimates the modelling provided to the government for its climate plan was not suitable for publication because had to be re-written "in an accessible language" for the general public.

The climate plan predicts 17 megatons of carbon will be stored in the nation's soil by 2050 - a figure The Australia Institute advisor Polly Hemming labelled "crazy" and most likely did not take into account the variability of land and the drying climate trend.

Modelling for the plan also predicted up to 63Mt of carbon could be offset each year by 2050 through 1.5 million hectares of on-farm plantings, or 43 tonnes per hectare.

However, Ms Hemming said even in the most fertile areas, trees reach a maximum sequestration potential of about 19 tonnes a hectare per year.

"I have no idea where these numbers came from, but it's just crazy," she said.

"Trees don't just infinitely sequester CO2, they reach equilibrium. To maintain a constant rate of sequestration you have to keep planting more trees. It's not a set-and-forget deal."

Under the 2050 roadmap, land offsets will account for 10 to 20 per cent of achieving the overall net-zero goal, which the agriculture industry has welcomed, pointing to the financial opportunities it would create for farmers.

But Ms Hemming said the "unrealistic plan" put pressure on farmers to be the silver bullet to offset fossil fuel emissions.

"Farmers are on the front line of climate change and many of them are already doing best practice when it comes to comes to soil," she said.

"This plan is an indictment on the government and it's letting [farmers] down."

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Greens agriculture spokesperson Senator Peter Whish-Wilson said once again the Coalition was putting the resource sector ahead of primary industries.

"By putting the interest of the LNP's fossil fuel donors before farmers, the LNP are not only selling out their political representation of farming communities but also selling out their future," Senator Whish-Wilson said.

"It's another betrayal, an insane and profoundly ignorant betrayal that the Nationals are prioritising a future of digging up and burning the exact dirty fossil fuels which will make parts of this country uninhabitable and unproductive in our lifetimes."

The Australia Institute chief economist Richard Denniss said the whole plan was "an exercise in imagining things get better over time."

"There's no consideration of what happens if anything in the model, not just soil carbon, gets worse - it's not just extreme techno-optimism, it's poor planning," Mr Denniss said.