The gang members or prospects involved were either victims or offenders.
Whanganui police area commander Nigel Allan said he couldn't say specifically what caused the rise in numbers because the figures included areas outside of Whanganui.
"Police and our partners work very hard to ensure community safety and well-being.
"We continue to target the negative effect organised criminal groups can have within our communities through disrupting the activity of these groups, holding offenders to account," Allan said.
Jarrod Gilbert, a sociologist and author of a book on the history of gangs in New Zealand, said changes in police strategy were often behind ups and downs in the violence figures.
"Just based on that data it's impossible to say why the increase and decreases have been happening and you'd have to look at the wider data to put it into context," he said.
"Often when you see increases that are out of whack like you see in Central it's not necessarily that violence is increasing but it may be that police have a specific operation going on ... where they're targeting the gangs for one reason or another.
"That can often lead to inflated figures, so not necessarily what's happening on the street but the policing of it."
Gilbert said it was likely police knew of specific operations where gang members were targeted.
"You get one or two quite big busts involving a lot of gang members and that can play havoc on data when you're only talking about 200-odd offences.
"One way that the police will target gangs if they're getting out of hand is to blanket police them - they target them specifically and put extra resources into them."
The Gang Intelligence Centre used a list of patched members and gang prospects known as the National Gang List to collate the figures.
The list is updated every two months with the best information police have at the time.
Police named 27 gangs, including Mongrel Mob and Black Power, as being responsible for the violent crimes.
