National leader Simon Bridges says the number of cases before the courts involving serious harm has increased by 25 per cent since the election in 2017.
He is basing his statement on answers to written parliamentary questions from Justice Minister Andrew Little to National's justice spokesman, Mark Mitchell.
Nationally, cases of serious harm were up 25 per cent, with an increase of 20 per cent in Auckland; 35 per cent in Waitakere; 32 per cent in Wellington; and 40 per cent in Christchurch.
The figures show that the number of serious harm cases before the courts in October 2017 was 17,189, that they steadily increased last year to 19,554 in December, and rose to 21,645 in April 2019.
The questions asked of Little was "What was the number of total active serious harm cases, broken down by district court, each month since October 2017?"
It is not immediately clear whether the increase is due to more crime or a bottleneck in the justice system, or both.
Neither the question nor the answer define "serious harm cases" but Bridges says they would include rape, sexual assault, murder, manslaughter and drug crimes.
Bridges said the statistics showed crime was increasing and cases weren't moving through the courts as quickly.
The increase is serious harm cases before the courts was also occurring at a time when the prison population was decreasing.
"The Government has taken its eye off reducing crime and is focusing on just getting numbers in prison down," said Bridges.
"I'm hearing from the frontline that youth offenders in particular are being caught by police for serious offending, and are sometimes back on the street within hours, committing more serious offences."
Slow-moving justice has the biggest impact on victims and that was not fair on people who had already been through serious trauma.
"As a former Crown Prosecutor, husband and father, I find these figures completely unacceptable.
"Our communities are now being directly affected by a Government that is more focused on reducing prison numbers than reducing serious crime in our communities," he said.
Andrew Little has not yet responded to a request for comment.