More than 45,000 people were arrested for minor offences such as drinking or fighting but given a warning rather than charged under a new system to cut the number of cases clogging courtrooms.
New figures show that 12 per cent of all arrests in the 2011/12 year were not prosecuted, according to an evaluation report of pre-charge warnings obtained under the Official Information Act.
The decision to issue a formal warning is at the discretion of senior police officers for anyone arrested for a crime with a penalty of no more than six months in prison, except in the case of family violence or methamphetamine.
The programme was implemented in September 2010 on the back of several Law Commission reports and calls from the judiciary to develop other ways to hold people to account for less serious offending without having to bring them before the courts. Since then, 45,836 warnings have been issued.
More than half of the warnings were for disorderly behaviour and breach of liquor bans, shoplifting, fighting and cannabis possession.
Police say the warnings have reduced the "unsustainable" pressure on the log-jammed court system and freed officers for more frontline work, instead of spending time preparing prosecution files for low-level offences.
However, the evaluation report highlighted the potential risk of officers issuing formal warnings rather than laying charges in order to cut back on paperwork.
Every single police district exceeded the pre-charge warning target of between 5 and 9 per cent of arrests in the 2011-12 year.
Waikato (18 per cent) Canterbury (16 per cent) and Eastern (14 per cent) used them the most. Counties-Manukau, Auckland, Bay of Plenty, Wellington and Tasman each resolved between 12 and 13 per cent of crimes, with Northern, Waitemata, Central and Southern between 9 and 10 per cent.
Mike Webb, acting general manager of police, said there were 21,881 fewer court cases in the 2011-12 which saved an estimated 37,000 staff hours or 21 staff.
The evaluation report noted that police had always had the discretion to give warnings but this practice had decreased by one-third over 15 years with an increased emphasis on arresting and charging for low-level offences under the "Broken Windows" theory.
This period also saw a high turnover of senior staff, which the report noted may have reduced the "transfer of knowledge and oversight in issuing warnings and less confidence among new officers on using discretion".
"Over time, this increased emphasis on prosecution has led to high demand on the court system, described as 'unsustainable', particularly in metropolitan areas with high volumes of minor offences."
The pre-charge warnings are still counted in the official police crime statistics, which recently showed that recorded crime fell by 7.4 per cent nationally in 2012, the lowest in 24 years.
The police are aiming to have reduced recorded crime by 13 per cent over four years by 2014-15 and reduce prosecutions by 19 per cent, which includes the warning initiative.
You've been warned
A pre-charge warning can be issued only to offenders who are 17 or older, have committed a low-level offence that carries a prison sentence of six months or less and who have admitted guilt.
Only an officer of sergeant rank or above can make decisions to issue warnings and they cannot be given for family violence or methamphetamine offending.
Police say 37,000 hours have been freed up for frontline officer time nationally, the equivalent of 21 fulltime staff.
There were 21,881 fewer court cases as a result in 2011-12 or 12 per cent of all arrests.
A total of 45,836 warnings were issued between September 2010 and last December.