Contract labour in Illawarra coalmines has surged massively in over year - at the expense of permanent jobs.
Coal Services - a joint body run by the industry and unions - records the number of mining jobs across the state's coalfields.
At first glance, the figures show improvement in the Southern Coalfields, which is made up of Illawarra mines.
The most recent employment data is for February 2019 and shows there were 2739 mining jobs in the Illawarra.
That was up from 2259 jobs 12 months earlier - a jump of 21 per cent.
However, it is where those extra jobs are coming from that has CFMEU Southern District Vice President Bob Timbs worried.
Between February 2018 and a year later, the number of workers directly employed by mines in the Illawarra rose from 1585 to 1625 - a paltry 2 per cent.
Over the same period, contract labour in the mines skyrocketed.
In 2018 there were 674 contract workers underground in the Southern Coalfields.
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A year later there were 1114 workers - a huge jump of 65 per cent.
Mr Timbs said coal mining jobs ebbed and flowed but historically, mine operators employed more staff in the good times.
"Coal mining jobs have always helped power this local economy, but that power has flowed in cycles," Mr Timbs said.
"What's different this time around is coal profits are up, but we're not seeing the accompanying lift in permanent jobs."
The union vice president branded the use of contract labor as "a cost-cutting exercise" for the mine operators.
"Some contracting companies have good union agreements with industry-standard pay and conditions," Mr Timbs said.
"But many contractors are now employed casually on much lower pay than workers employed under site agreements, with no paid holidays or other entitlements."
He said contracted workers deserved annual and sick leave as well as job security.