Drug busts have been mistakenly double-counted for the past seven years, and some experts warn the exaggerated data might have been used to justify crackdowns on drug users and support bad policy.

NSW Bureau of Crime Statistics and Research BOCSAR director Dr Don Weatherburn admitted his bureau was at fault, and said the drug bust mistakes built gradually from 2010.

Cocaine and ecstasy possession reports to police were inflated more than 30 per cent in recent years, while last year in total, 13,350 recorded drug use/possess events never happened.

“When we saw they [NSW Police] were doing searches and they were proving positive [drug detection] we just assumed they hadn't recorded that positive result, so we added it in ourselves. That was a mistake because they were adding it in,” Dr Weatherburn said.