Emergency services were called to the incident at Cobar, some 450km east of Broken Hill, at about 3.45am on Tuesday.
Police were immediately told one man had been confirmed dead after the underground explosion.
Two women were then brought to the surface, but one of them died shortly after.
The second woman was airlifted to hospital suffering hearing damage and shock.
At about 7.15am, police confirmed the deaths and said emergency services remained at the scene.
SafeWork NSW will lead an investigation into the incident, with police to also prepare a report for the coroner.
Copper and gold mining are the two leading employment industries in the region.
The mine is a major underground silver, zinc and lead mine, 40km north of Cobar.Â
It's the second largest project in the polymetallic region known as the Cobar Basin.
The mine was acquired by Polymetals in 2023 and planned to restart mining activities earlier this year.
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