Issuing warnings for cannabis possession "effectively decriminalises" the drug, a campaigner says.

Under an Auckland police trial, warnings can be given for offences that carry less than a six-month prison term.

Eligible offences include shoplifting, disorderly behaviour and low-level cannabis possession.

The three month trial in the North Shore police area has been extended to the greater Auckland region for a further six months.

Canterbury's police district commander, Superintendent Dave Cliff, said Christchurch would consider the Auckland results before deciding whether to introduce a similar approach.

Waitemata police district commander Bill Searle said of those arrested during the trial period, 10 percent were warned rather than charged.

In the same period, the number of diversions for low-grade first offences granted in the district court fell 64 percent.

However, anti-drug campaigner and former police officer Mike Sabin said the approach "waved a white flag" to criminals.

"in essence, this signals decriminalisation of what many would argue is the foundation block of our nations drug problems," he said.

"to give up on so-called minor offences is like a fat man letting his belt out to sober his obesity problem."

Sabin said, in reality, officers would stop enforcing drug crime if they knew offenders would only receive warnings.

"Imagine someone smoking dope outside a nightclub at 2am. Can you imagine them being taken back to the station to be formally warned?"

"It's just not going to happen."

Searle denied it was a soft approach. Police officers were still required to make arrests and the decision to warn rather than charge was made by a senior officer, at the station, under guidelines.