"It's like saying petrol is going to cost you $10,000 this year by assuming you buy everything in one go on Boxing Day."

The government estimates it must cut 328 million tonnes of carbon by 2030 while Labor must cut 1.3 billion tonnes to achieve its more ambitious climate change target without using"carry-over credits"from past United Nations agreements.

Mr Morrison has estimated twice in recent days that Mr Shorten's commitment would require companies to spend $35 billion on international carbon permits.

"Now,one of the questions he won't answer is what proportion of that 1.3 billion tonnes is going to be satisfied by buying foreign carbon credits?"Mr Morrison said on Saturday while campaigning in western Sydney.

A Coalition spokesman said it was up to Mr Shorten to explain how his policy would work and how many foreign permits employers would have to buy to meet his carbon target.

"For weeks now,Mr Shorten has been unable to explain what his emissions reduction policy will cost,or the detail of how they'd meet their targets,"he said.

The Coalition is preparing to release further modelling from BAEconomics director Brian Fisher in the coming days to claim a major impact on the economy from the Labor target of reducing emissions by 45 per cent by 2030 compared to the government's 26 per cent.

Concerned at the use of taxpayer funds to commission Dr Fisher's modelling,Mr Shorten wrote to Dr Parkinson last week to ask who had sought the analysis of Labor's policy.

Dr Parkinson replied on Wednesday with an assurance the public service had not costed Labor's policy but that the Department of Industry,Innovation and Science had been asked in January to seek"external modelling"of a potential emissions policy. This is believed to be Dr Fisher's analysis.

"The limitations of modelling are well known,"Dr Parkinson told Mr Shorten in his letter.

"As such,I would regard it as misleading to release the results of this modelling exercise without also acknowledging both the underlying assumptions and the manifest limitations."

The government assumes the Labor policy would require companies to buy credits for 650 million tonnes of carbon at $53 per tonne,producing a cost of $34.5 billion over a decade.

Bloomberg NEF has made no estimate of the international permits required under the Labor policy and expressed caution about projecting so far ahead.

"The most accurate answer to the question of how much money will flow offshore from the purchase of overseas carbon credits is we don't know,"Mr Bhavnagri said.

"We also don't know what percentage of the required emissions reduction Australian businesses will find it cheaper to go overseas for. It could be cheapest to do it all here."

Energy Minister Angus Taylor has cited earlier work by Dr Fisher to claim a $1.18 trillion cost from the Labor emissions target,calculated by assuming the cumulative losses in economic output over a decade. This a very high cost given the annual output of the Australian economy is about $2 trillion.