Documents obtained through a Freedom of Information request by climate action group Lock the Gate Alliance show two exploration licences could add 15 years to the life of the Whitehaven operation.
But the NSW government says the granting of licences doesn't mean the mine will necessarily continue producing coal beyond its current approval window.
Originally slated to operate until 2031, Whitehaven was in April granted an extension to 2044 to extract a further 82 million tonnes of coal.
The Independent Planning Commission said the project on the state's northwest slopes fulfilled an "appropriate balance between relevant environmental, economic and social considerations".
The underground mine has been operating since 2011 and accounted for two to three per cent of the state's saleable production of thermal coal in 2019/20.
Whitehaven said the 13-year extension granted in April would provide the state with $600 million in economic benefits and keep 500 local jobs alive for decades.
However, the exploration licence documents show an expansion of the mine into two areas to the northwest could extend the life of the coal project by another 15 years to 2059 if the sections were approved and developed.
The potential extension of the Narrabri mine beyond 2044 has been lambasted for going against the government's commitment to achieving net-zero emissions by 2050.
"Behind closed doors, the Perrottet government is waving through coal proposals that would make it impossible for the state to reach its existing carbon reduction goals," Lock the Gate national coordinator Ellen Roberts said.
"There is no way the government could reach net zero by 2050 if Whitehaven continues to expand thermal coal mines until 2059."
If developed, the mine extension would also boost production from 5.4m tonnes per year to 8m tonnes, the documents say.
"The Perrottet government needs to stop sacrificing water and farmland for an industry that is the main driver of dangerous climate change," Ms Roberts said.
The exploration licence applications were submitted in February and approved by Deputy Premier Paul Toole in September for an area covering 3721 hectares.
Both applications are for land in an area known as Gorman North, which the government has identified for potential strategic release.
Local farmers oppose the move, arguing the area sits atop a recharge zone for the Great Artesian Basin and mining thermal coal threatens the ongoing supply of groundwater vital for local agriculture.
Mr Toole downplayed the fears, saying that exploration did not necessarily mean the areas would be mined.
"The grant of two coal exploration licences in Narrabri authorise exploration activity only – they do not authorise mining, and the granting of an exploration licence is no guarantee that mining will take place," he told AAP.
Last year, Bushfire Survivors for Climate Action launched a legal challenge to the planning commission's approval of the Narrabri mine operations until 2044.
A court hearing is scheduled for January 31.
