A drug detection company says most of the positive results it gets in workplace testing are for cannabis.
New Zealand Drug Detection Agency chief executive Kirk Hardy says tests in the South Island also show up opioids such as heroin, morphine and codeine, and in the North Island stimulants such as methamphetamine and cocaine.
However cannabis is the drug detected 70% of the time.
Mr Hardy says the use of the drug is far from innocuous as it prevents people concentrating, multi-tasking and accurately judging time and distance.
He says that can have catastrophic effects, particularly in sectors such as construction and forestry.
After random drug testing was introduced to the forestry sector in 2008, Mr Hardy says, accidents fell by 36% in two years.
He also says regular smokers of cannabis have a higher chance of developing schizophrenia.
And Mr Hardy says a 2008 study showed regular cannabis smokers who developed lung disease did so, on average, 24 years earlier than tobacco smokers.
The agency conducted 77% more worksite tests in 2011 than the previous year.
Tests returning a positive result fell to 7% in 2011 from 9% the year before.