Bathurst Resources' open cast coal mine on the Denniston Plateau north of Westport is likely to close.
The mine supplied 60,000 to 80,000 tonnes of coal annually to the nearby Holcim cement plant.
But Holcim's Swiss owners decided some time ago to shut the plant down, and would formally cease operations in June.
Bathurst Resources chief executive Richard Tacon said his firm's deliveries would stop in April
Without the cement plant the 20,000 tonnes of coal it mined annually was not consumed by Holcim was not enough volume to make operations worthwhile, he said.
He said 18 of 23 staff were likely to be laid off.
Mr Tacon said staff continued to look for markets but he had to be realistic about chances, although he had told staff not to get their hopes up.
The site at Denniston would be retained and could be revived if coal prices jumped up again, he said.
Bathurst Resources would continue mining in Southland and Canterbury, Mr Tacon said.