While the IPC has discretion under "exceptional circumstances" to refuse United Wambo mine's expansion plans because of downstream emissions, the discretion has to be exercised carefully within the "broader policy framework", Mr Betts said in a letter to the IPC on Thursday.

He advised the IPC there was "no policy at either the state or commonwealth level" to support imposing a consent condition requiring the Glencore/Peabody joint venture to ensure "all practicable measures" are taken to minimise emissions in countries where its coal is exported, including Japan, China, South Korea and India.

He advised Tony Pearson, the chair of the IPC panel considering the Singleton mine expansion, that it was "not this state government's policy that greenhouse gas policies, or planning conditions, should seek to regulate, directly or indirectly, matters of international trade".

"Any such policy is likely to result in significant implications for the NSW and Australian economy and it is not clear it would have any effect on reducing the global greenhouse gas emissions generated by parties in other jurisdictions outside Australia," Mr Betts wrote.

"Even if such a policy was made, it is likely to be more efficient and equitable to apply it across the board through legislation rather than waiting for individual companies to apply for development consent under the planning system in NSW."

Mr Betts responded after the IPC called for submissions as it nears a final decision on the Glencore/Peabody plan to expand and mine up to 150 million tonnes of additional coal. It would double the site's existing workforce of 250 and generate an estimated $820 million in royalties, the IPC was told.

It proposed requiring the Singleton joint venture project to prepare an export management plan linking the sale of Australian coal to countries with policies to reduce greenhouse gas emissions in line with the Paris Agreement.

The agreement includes an international goal to hold global warming to "well below 2°C" and to pursue efforts to limit the temperature increase to 1.5°C above pre-industrial levels, requiring countries to achieve net zero emissions as soon as possible after 2050.

Mr Betts' response was published on the IPC's website as Prime Minister Scott Morrison was in Tuvalu where low-lying nations at a Pacific Leaders Forum called on Australia to stop new coal mines and take stronger action to reduce greenhouse gas emissions.

The Department of Planning intervention also followed a strong backlash to the proposed condition from the mining industry and NSW Deputy Premier John Barilaro who accused the IPC of "judicial activism" and "over-reach".

Australian Minerals Council chief executive Tania Constable described the proposed condition as an "over-reach" that would "undermine good management of Australia's foreign trade" by requiring the Hunter mine to manage "emissions of foreign sovereign nations".

Ms Constable acknowledged its intervention was highly unusual because "as a national body the Minerals Council of Australia does not usually comment on individual state-based approvals".

But the IPC proposal may be "damaging and counterproductive" and potentially undermine the Paris Agreement by distorting it, Ms Constable said.

"It is neither practical nor plausible for a state government agency to determine or manage the responses of other nations," she wrote.

Lock the Gate Alliance spokesperson Georgina Woods described Mr Betts' submission as an "extraordinary intervention from the Planning Secretary" that could discourage the IPC from imposing the condition.

Ms Woods accused the NSW Department of Planning of "failing its duty to consider the serious and lasting consequences of expanding coal mining", and said the IPC was one of the only authorities in the state "trying to take its responsibilities on climate change seriously".

"The Secretary argues the IPC's power to refuse approval to a coal mine on the grounds of its contribution to climate change should only be wielded in 'exceptional circumstances'," Ms Woods said.

"Most people consider our current circumstances exceptional given extreme weather, record heatwaves and unprecedented drought is devastating lives in Australia while state and federal governments fail to reduce carbon pollution in line with the Paris Climate Agreement.

"The only useful element of the department's submission is that it acknowledges the IPC has the power to refuse approval of a coal mine on the grounds of its downstream greenhouse emissions.

"If it's too hard to craft a condition that can ensure these emissions are minimised, the IPC should just refuse the project altogether, especially in light of the cumulative impacts it will have on air quality and public health in the Hunter."

In his submission Mr Betts referred to the landmark Land and Environment Court decision backing NSW Government refusal of the Gloucester Rocky Hill mine which "noted that scope 3 emissions (where Australian coal is burnt in other countries) "are able to be taken into consideration in determining whether or not to grant consent to a development application".

But while the Rocky Hill case established the point, "it does not automatically follow that scope 3 emissions are properly the subject of a condition, are warranted in any particular case, or that a particular condition that refers to scope 3 emissions is necessarily appropriate", Mr Betts said.

"Every condition of a consent must be for a proper planning purpose, must fairly and reasonably relate to the subject development, and must not be manifestly unreasonable," he advised the IPC.

In a submission to the IPC Australian climate expert Professor Will Steffen said it was an "absolutely clear, scientifically indisputable fact" that if the global average temperature rise is to be limited to "well below 2 degrees Celsius "then expansion of the fossil fuel industry must end immediately".

Professor Steffen said countries should be embarking on a two to three decade phase-out period for fossil fuel usage so that greenhouse gas emissions reach net zero by 2050.

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