A FORENSIC investigation is under way into allegations an analysis of coal samples was amended to make the product look better quality than it was, before it was shipped offshore.

Lab testing giant ALS has appointed external advisers to conduct a forensic probe of the coal samples process after discovering an unspecified number of analysis certificates had been changed "without proper justification".

It's believed at least two of the three coal testing laboratories - at Gladstone, Mackay and Newcastle - were involved.

Four members of its coal unit were suspended on Monday pending the outcome of the investigation.

The $4.6bn company was made aware of the issue by an unfair dismissal dispute lawsuit which accused it of being involved in the scheme with mining company TerraCom.

The litigation, filed by former commercial general manager Justin Williams, against six of TerraCom's directors alleges he was dismissed from his role after rejecting demands by chief executive Danny McCarthy and chief financial officer Nathan Boom to make ALS change its analysis results.

The Australian reported that in his application Mr Williams estimated up to $195.7 million worth of coal shipments could have been affected.

In an announcement to the Australian Stock Exchange, TerraCom, which is not a respondent in the case, "vigorously" denied the allegations.

The company said it took the allegations "extremely seriously" and had the conduct of its employees independently investigated.

It noted the claim was made by Mr Williams after he was made redundant - one month after he started work and within his probationary period.

It said coal exported from its Blair Athol mine in Central Queensland was subject to three stages of independent testing: when the coal arrived at Dalrymple Bay Coal Export Terminal, on-board the vessel and by Japanese and South Korean customers when it landed on the dock.

Meanwhile ALS said it had to stop the amendment of certificates without proper justification.