The percentage of Northland court cases ending up with convictions is higher than at any time in the last 35 years.

Statistics New Zealand figures reveal more than 88 per cent of adults prosecuted in Northland courts in 2015 were convicted, the highest percentage since data became available in 1980. This figure has increased in five of the last six years.

Whangarei lawyer Wayne McKean said more people decided to plead guilty and that had contributed to the rise.

He said changes to the payment system for legal aid lawyers may also provide a financial incentive to avoid trials.

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"You've got defendants represented by lawyers who are unlikely to get paid adequately if they take the case to trial," Mr McKean said. "What it means is, people are pleading guilty to offences that they otherwise wouldn't plead guilty to."

Mr McKean said another factor was police were sometimes laying charges with heavier consequences as "leverage".

"Police lay more serious charges than they otherwise would, and then use those serious charges in the plea bargaining as a carrot to the defendant. If he pleads guilty to the lesser charge, they'll withdraw the more serious charge."

A 2011 review of public prosecution services said three national law bodies - including the New Zealand Law Society and Criminal Bar Association - expressed concerns at police and crown solicitors using "inappropriately serious charges". The police acknowledged this occurred but said it was usually due to inexperience.

Mr McKean said a backlog of court cases around the country also encouraged judges and prosecutors to resolve cases without trial.

"It's a huge concern for me that it's happening. I think people charged with offending are entering a system which heavily incentivises the defendant to plead guilty."

Police refused to comment on figures by Statistics New Zealand but said Police Prosecution Service data showed Northland's successful case outcomes such as conviction, discharge without conviction, and diversion rates have remained static over the past five years.

A spokesperson said its prosecution service staff worked under the Solicitor General's guidelines and made objective case-by-case decisions based on relevant evidence at hand. "In any matter before the court, individuals have the right to defend or challenge the charges presented by the Crown. How they choose to defend charges is a matter for them and their legal counsel".

Percentage of convictions from prosecutions:

Northland district courts (Kaitaia, Kaikohe, Whangarei, Dargaville)

* 2009: 82.1

* 2010: 83.7

* 2011: 85.1

* 2012: 86.4

* 2013: 86.3

* 2014: 87.0

* 2015: 88.5

Source: Statistics NZ