The miner, owned by British billionaire industrialist Sanjeev Gupta's GFG Alliance, had last week talked up its future workforce prospects, saying its workforce of 400 would be boosted by another 175 if it were to get planning permission for the next stage of the mine's life.

But shortly before these claims were being made, SIMEC had actually laid off 63 workers with a quick phone call.

A mine source said the calls happened on February 18, and said workers were told by management that 105 jobs were being cut.

"People got a phone call, they said they'd chosen who they want, told them they were on a list," he said.

"A week or two later they got a letter saying it was because of uncertainty in the industry."

But a GFG Alliance spokesman said the figure was certainly 63.

"We are excited about the future of the mine with the proposed Tahmoor South extension," the spokesman said.

"If approved, the extension will provide ongoing employment for our 400 permanent employees for a further 13 years, create 175 new jobs during construction and deliver $162 million in economic benefit to the region."

He said they had been working on underground work in preparation for linking to the new Tahmoor South area.

"Obviously it's never ideal when contractors are impacted, but we've recently had to adjust our work schedule to match underground development activity with the approval process timeline for the Tahmoor South extension."

"Our contracting workforce complements our 400 permanent employees on site as needed. There is no impact to our permanent employees."

The jobs lost were all contract workers engaged by Bargo company RStar.

The mine source said some of the workers let go were working on development on the Tahmoor North area - which he said wasn't anything to do with the new expanding section.

"That's a lot of blokes to find somewhere to go to [for a job], 65 in the Illawarra at the moment," he said.

Planning authorities are considering the expansion proposal.