An analysis by Lock the Gate shows the "loophole" is allowing coal expansion projects, which plan to emit hundreds of millions of tonnes of greenhouse gases, to be green-lighted without a determination by the Independent Planning Commission (IPC).

This means there is no public hearings, and a much less detailed level of assessment conducted internally by the NSW Planning Department.

According to Lock the Gate analysis based on publicly-available data, provided by mining companies, at least nine of the 17 coal mine expansions currently on the cards are being presented as modifications.

Some of those allow coal companies to dig entirely new pits, while others are digging deeper, and for longer.

Only four of those nine "modifications" plans provide estimated greenhouse gas emissions figures - they come to a total of 324 million tonnes of life cycle CO2-e emissions.

That is equal to two and a half times the volume of emissions produced in NSW in 2019, from just four coal mines that are not subject to independent scrutiny, Lock the Gate says.

Lock the Gate Alliance NSW Coordinator Nic Clyde said it was unacceptable for the NSW Planning Department to make decisions about "such massive coal mine expansions" without the transparency and community engagement that the Independent Planning Commission affords.

"Before it was elected, NSW Labor committed to assessing coal projects independently," Mr Clyde said.

"Lock the Gate's analysis shows that for half of the new coal expansions being proposed, that's not happening.

"It's also difficult to see how the NSW Minns Government can make good on its new emissions reduction targets while simultaneously waving through new polluting coal expansions and extensions.

"We're really concerned that big coal companies are using the modification loophole to incrementally expand coal mines when larger expansions were previously rejected by the IPC, or where they face unwavering community opposition.

"This is the expansion of the coal mining sector by stealth - it's a bonafide coal mod con on the NSW public.

"The International Energy Agency has warned that no new fossil fuel projects or expansions can be built if the world is to maintain a safe climate.

"For these new coal projects to be sneaking through the system without independent scrutiny and transparency is an insult to NSW communities who are already grappling with the impacts of climate change."

A spokesperson for the NSW Department of Planning, Housing and Infrastructure said every new mining proposal and expansion in NSW, along with modifications to approved projects, undergo a "rigorous assessment process" that considers potential impacts to the surrounding environment and community, which are "thoroughly investigated".

The department said a decision on whether a planning application should be a modification to a development, or a new state significant development, depends on whether a modification is substantially the same development as originally approved or last modified.

In an emailed statement, the department provided further information on background:

"While there is no strict planning threshold as to what would constitute a modification to a development, there are principles based on case law that inform the appropriate planning pathway," the statement said.

"The assessment of all new mining projects and modifications will also be considered against contemporary greenhouse gas policies and guidelines including the NSW Government's targets to reduce net greenhouse gas emissions by 50 per cent by 2030, 70 per cent by 2035 and net zero by 2050, and the Strategic Statement for Coal Exploration and Mining, which sets out its approach for transitioning to a low carbon future."

A spokesperson for Peabody, the coal mining company behind one of the nine case studies put forward in the Lock the Gate analysis over its plans for a mine in Wollongong, said "all modification applications are made in accordance with the NSW Government's robust planning and approval processes".

Similarly, a spokesperson for the Bloomfield Group, which is approved to continue operating the Blooomfield Colliery Continuation project at East Maitland until 2030, said their "assessment methodology" has been approved.

"The continuation of mining does not involve an increase in annual production, additional equipment or additional infrastructure."

Glencore, which operates Mt Owen/Ravensworth at Singleton, with plans in to mine another nine million tonnes of coal producing 2.7 million tonnes of greenhouse gases, did not reply to a request for comment.

MACH Energy, which operates Mount Pleasant at Muswellbrook, where there are plans to mine an additional 57 million tonnes of mine under a modification application, taking the total to 192 million overall, formally declined to comment.

The Newcastle Herald received no response to questions put to other coal mining companies with modifications in the wings named in the report, including BHP in relation to its Mt Arthur mine at Muswellbrook.