Defence lawyer Louise Elder said the pre-charge warning system could be inconsistent.
"The process needs to be transparent because ... there are regional variations and differences of opinion with the people who are making these decisions and so my concern is that people in one area might get a pre-charge warning and someone in a different area might get a charge," she said.
"That might even happen within the same police station. We just don't know how that discretion is exercised."
Ms Elder had dealt with clients who would have been "perfect" for the scheme, but were charged anyway.
"I'm scratching my head as to why this hasn't been dealt with a pre-charge warning."
The warnings had cut court traffic significantly, but there wasn't less crime, she said.
"It's just being dealt with differently."
A spike in arrests over "National Crate Day" weekend in December saw one Masterton man slapped with a pre-charge warning for disorderly behaviour.
The previous year, six men aged between 17 and 20 were arrested and given pre-charge warnings when they were found unlawfully on a Masterton building site.
In June 2012, a woman who braved sub-zero temperatures to run on to the field naked in freezing weather during an All Blacks game in Christchurch was let off with a pre-charge warning instead of being charged with disorderly behaviour.
Police issued 20,583 pre-charge warnings nationwide in 2013, down substantially from 24,341 the previous year but up from 17,446 in 2011.
The most common offence was disorderly behaviour in public, followed by theft and illicit drugs.
The system means police can arrest a person and issue a warning as an alternative to a charge or prosecution.
A warning is not a criminal conviction, but still goes on the offender's record and is considered if they commit another offence.
It is included in published crime statistics, but won't show up when employers search for a prospective employee's criminal history.
"The initiative particularly targets crimes often committed by offenders who are intoxicated, and many of whom have not previously offended," a police spokesman said.
"The lengthy and costly process for prosecuting can be seen as disproportionate to the circumstances, especially if it results in a conviction with implications for the individual's future."
While offenders could be seen as being "let off" when given a warning, the process sent a stronger deterrent message than an informal warning because offenders were arrested and taken to a police station, the spokesman said. APNZ
