The press release was straightforward: the data showed drug offences were up more than 31 per cent and therefore the government would devote more resources to the war on drugs.

"The O'Farrell Government is committed to establishing a second Sydney Drug Court and a Metropolitan Drug Treatment Facility stopping people from becoming caught up in the drug crime revolving door," the NSW Police Minister at the time is quoted as saying in the April 2011 release.

The only problem is the data was wrong.

The Bureau of Crime Statistics and Research (BOCSAR) has admitted to double counting more than 80,000 drug crimes in the state over nine years.

In some years, the published total number of use and possession offences was inflated more than 30 per cent over the correct figure.

In the twelves months to March 2011, a month before this release, BOCSAR had mistakenly inflated the number of drug use and possession offences by 7,096, or more than 30 per cent.

This showed that drug offences were up 31.7 per cent on the previous 12 month period, when in fact they were only up 8.9 per cent - one example of how false drug offence statistics may have created a false perception of a drug crisis in New South Wales, and skewed the allocation of millions of dollars of police resources towards fighting drug crime.

The mistakenly inflated data has been used to back up the use of drug sniffer dogs at music festivals as well as other drug-related police strategies.

In the war on drugs, trust in the official crime stats has just taken a hit.

How the mistake became public

The mistake was made public after the crime stats bureau posted a small notice on the front page of its website: "The number of recorded drug possession incidents reported in years 2010 to 2017 has been revised down due to double counting of some drug possession incidents that came to police attention through a person search."

That perhaps did not describe the magnitude of the error.

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Whatsapp Historic versus revised total drug statistics from April 2008 to March 2018.

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Whatsapp Total drug crimes historic and revised data for different classes of drugs.

BOCSAR director Don Weatherburn told Hack the mistake in the raw number of drug crimes was "very substantial".

"It did affect the trends," he said.

"There would have been about eight [quarters] when we thought the trend was up when it was stable and and maybe four or five more where we thought it was stable but it was actually down."

Dr Weatherburn said there was "absolutely" no intention to fudge the data.

"I understand people's suspicion on that front but this was just a simple if greatly regrettable error," he said.

There was no conspiracy.

"The bureau is here to give the public confidence they have access to independent and objective, reliable crime statistics."

"My first reaction was terrible disappointment that we have misled people."

Although the mistake skewed the quarterly trends, the both the incorrect and correct figures show a long-term increase in drug use and possession.

How the mistake happened

The mistake was a case of double-counting, BOSCAR said.

The bureau was adding the figure for the number of use and possession offences to the number of positive drug searches, not knowing that police had already done this.

Dr Weatherburn said this was a "failure of communication" due to both agencies trying to keep comprehensive drug crime records.

"The logic is we didn't want to miss out on any drug offences," he said.

"We were creating drug offences to make sure we caught everything."

Did it skew policy?

Dr Weatherburn says probably not, while criminologists disagree, saying policy and policing decisions are based on BOCSAR figures.

NSW Greens MP David Shoebridge, a strong advocate of winding back the tough on drugs policy, told Hack police should have picked up on the error.

"If the police weren't aware the official data of one of the most highly observed parts of their activity was out by about by 30 per cent, then this is a significant problem."

[The mistake is] good for police, it makes them look like they're doing more than they did.

"They have used this kind of data about prevalence of drug use to expand the drug dog program across the entire public transport system."