Coal workers say COVID-19 is leaving them short on good masks, with a number of fakes turning up on worksites.

Coal miners use P2 and N95 masks to protect against dust-borne diseases like black lung.

But union figures say high demand has led to fake and substandard face mask being worn at some mines.

“We had masks coming in that weren't rated and were actually ineffective and fake at our coal mines,” CFMEU spokesperson Steve Smyth has told the ABC.

“This is a huge concern because there are guys relying on these masks each and every day, thinking they're effective.”

The Queensland Department of Natural Resources Mines and Energy has tested masks at one underground coal mine and found 40 per cent had a 0 per cent pass rate for facial seals.

Workplace Health and Safety Queensland says it can be very difficult to spot a fake or unaccredited mask, as many are marked as P2 or come with packaging indicating they have met testing requirements.

WorkSafe New South Wales has found that documentation is sometimes being provided by unaccredited testing facilities with no licence numbers.

Additionally, some companies use the approval number of accredited companies, without their permission.

“Luckily the guys themselves started to realise there was something wrong with these masks,” Mr Smyth said.

“They were packaged the same, but they were fake, it's very deceptive.

“It's a failure of the system that these were allowed to get in, but again, it's people taking the opportunity off the back of a pandemic.

“They were selling them knowing these masks were ineffective and not should be used in coal mines in Queensland.”