Solid Energy's random drug and alcohol tests nabbed 28 offenders in the year to July, more than a third less than the previous year.
Fifteen were Solid Energy employees, equating to about 1.4 per cent of Solid Energy's 1036-strong workforce at June 30, according to information obtained under the Official Information Act.
Thirteen worked for contractors.
Six of the 28 had offended before. Three of them subsequently resigned and three were dismissed, said Solid Energy senior solicitor Rob Page.
Another 17 were either dismissed or were no longer working at the sites and 11 were referred to a rehabilitation programme.