Whangarei lawyer Wayne McKean said the judicial system "heavily incentivises" defendants to plead guilty, affecting the conviction rate.
One reason was prosecutors over-charged defendants to increase their strength in plea-bargaining.
"If a defendant pleads guilty to the lesser charge they'll withdraw the more serious charge," Mr McKean said. "It elevates the risk to the defendant if he doesn't accept it.
"I think that's wrong, using more serious charges as leverage."
The Law Society and the Criminal Bar Association had expressed concerns at police and Crown Solicitors laying "inappropriately serious charges" against offenders. The police acknowledged this occurred but said it was usually due to inexperience.
The rise in conviction rate coincides with the lowest number of people going through court nationwide since at least 1980. The figure has dropped almost 40 per cent since 2009 - from 127,000 prosecutions to fewer than 77,000 nationwide.
Labour's police spokesman Stuart Nash said he was concerned about the sharp decrease. "It says to me that the police just haven't got the resources to catch the bad guys.
"The last thing we want ... is to have any sort of questioning of the validity of our justice system."
Lawyer Stephen Ross said the downward trend in prosecutions was due to police cost pressures.
"It is a budget issue ... There's no doubt about that," he said.
In a written response to NZME, police said their baseline budget had not been frozen, and their operating funding would increase $41 million a year over the next four years.
Police were making operational changes around the country "to serve their communities better", but there had been no cuts in staffing.
A 2012 Ministry of Justice report stated a crime reduction target of 15 per cent between 2011 and 2017. But since 2011 prosecution and conviction numbers have fallen 28 per cent and 27 per cent respectively.
The Police Association said a police policy of warning offenders for low-level crimes had gone too far.
"Police have way overshot," said president Greg O'Connor. "I don't think anybody believes there are fewer people offending or that there's less low-level offending taking place."
He was unsure how the long-term effects of such warnings would balance out against the short-term gains.
"If you are the Government and want to ... say the number of arrests have gone down and the number of court appearances have gone down, then yes, it's a great policy."
"A spokesperson for Police Minister Judith Collins said the higher conviction rate was a result of fewer prosecutions. The spokesperson said the rate of successful case outcomes - conviction, diversion and discharge without conviction - had been "relatively steady" in the last four years, increasing 1.2 per cent since the 2012/2013 financial year."
Convictions
as a percentage of adults prosecuted in New Zealand
2015 83.2 per cent
2014 83.5 per cent
2013 82.7 per cent
2012 81.8 per cent
2011 81.7 per cent
2010 79.8 per cent
2009 78.5 per cent
Source: Statistics New Zealand
