Environmental groups want more time to consider the impacts of Santos' proposed $3 billion Narrabri coal seam gas project that includes as many as 850 wells, and would produce more than 100 tonnes of salt alone per day at its peak.

The 7000-page environmental impact statement for the project in north-west NSW was finally revealed on Tuesday, with the state government placing it on exhibition until April 24, "given the high level of public interest".

But Sue Higginson, principal solicitor of the NSW Environmental Defenders Office, said Santos had years to develop the "enormous" statement and the public needed at least 90 days to be able to test its many technical claims.

"We haven't seen anything of this scale in this state," Ms Higginson said. "This will have long, long, long-term impacts."

The planning department said the extension request was being considered. As part of the broader project assessment, the department will set up "a panel of eminent scientific experts" and will conduct "in-depth analysis of all issues raised from the public consultation", with no deadline set, a spokeswoman said.

The energy giant, which last week announced it lost more than a billion dollars in 2016 and has also flagged a possible exit from CSG in NSW, said the project could supply as much as half of NSW gas needs. It would also generate about 1300 jobs during construction and 200 during its operational life.

Santos said the 850 wells would be progressively drilled over 20 years, with the highest rate during the first three to four years of the project. The venture will spread over 950 square kilometres in and around the Pilliga state forest, and tap into coal seams mostly 500 to 1200 metres underground.

"There is between 250 and 400 metres of relatively impervious rock that isolates the target coal seams from the shallower aquifers used by agriculture and the community," the company said, adding the region's "favourable geology" also means gas can be extracted without hydraulic fracturing, or fracking.

Some residents, though, will take some convincing, with the Northwest Alliance raising concerns about potential impact on the Great Australian Basin as well as the prospect of other future CSG projects.

"We know that CSG in the Pilliga is a trojan horse to access vast areas of agricultural country in north-west NSW," Megan Kuhn, a grazier on the Liverpool Plains, said.

"Santos has announced plans for seven major gasfields across our productive farming region. They want to replicate the Queensland [CSG] disaster on us but clearly lack a social licence, which is necessary for them to begin."

Among the environmental issues is salt, which will peak at 115 tonnes a day during the second to fourth years of the project, and average 47 tonnes daily over 25 years, Santos said.

"The salt would be managed by off-site disposal to an appropriately licensed facility," the company said.

Stuart Khan, an associate engineering professor at the University of NSW, said Santos had previously stated dumping the salt into landfills was a last resort as it hunted commercially viable "beneficial reuse" for the salt, from glassmaking to sodium bicarbonate production.

"But here we are proposing to landfill 430,000 tonnes of salt over the life of the project," Professor Khan said.

"Landfills will be lined with synthetic liners to prevent leakage to groundwater, but all synthetic liners have a lifespan and they will all fail over a period of time," he said. "In time, it is certain that someone will need to come along and clean up these legacy salt landfills in order to prevent environmental damage."

Fairfax Media sought comment from Planning Minister Anthony Roberts about an extension of the consultation. Labor said it remains opposed to the Narrabri project and supports the longer exhibition period.

"The community has been battling coal seam gas for many years now and attitudes have only hardened against it," Jeremy Buckingham, the Greens energy spokesman, said. 

"We've seen spills of toxic water kill large swathes of forest, the pollution of an aquifer with uranium, and leaking infrastructure from only a few gas wells in the exploration phase of this project," he said.

"If this project is approved, it will multiply this damage and risk with 850 gas wells, associated pipelines, powerlines, compressor stations and water treatment plants."

• Separately, the NSW government on Tuesday appointed the state's chief scientist Mary O'Kane as chair of its new Energy Security Taskforce to examine how to improve management of extreme weather impacts on the state's infrastructure. This taskforce follows a record heatwave earlier this month that strained the state's electricity sector.