Health and safety services in Christchurch say they're starting to see fewer rebuild workers testing positive for methamphetamine.

TriEx testing saw a spike in positive tests in construction workers around four years ago, as the recovery began to peak, but this year they're finally seeing the rates start to drop in the city.

Clinical Manager Karen Woolf said employers have educated their workers about the dangers of the drug, and this year there's been fewer positive urine tests.

"A lot of employers, certainly within the construction industry, know a lot more about methamphetamine than we did 3-4 years ago. Prior to that time, we never really saw it."

But meth can pass through the body in 24 hours - and she said workers may be doing a better job at hiding it.

"It is a concern because it does get metabolised very quickly in the body."