Number of imprisoned gang members exploding in New Zealand

The number of gang members in New Zealand's prisons is exploding and it's causing concern for Corrections officers.

About 38 percent of inmates are gang members or affiliates - more than 3500 people. But over the past few years, some gang numbers have dramatically risen. The Mongrel Mob, Tribesmen and Bloods are all up by at least 60 percent, while Head Hunters is up 150 percent and the Killer Beez has increased 126 percent.

Neil Beales from Corrections says that's causing trouble.

"We're seeing more violence. We're seeing more violence perpetrated by gangs, particularly in these younger gangs that are coming through," he told Newshub.

On the outside, gang numbers are ballooning too, nearly doubling in five years. The National Gang List now covers 27 groups.

Deportees from Australia like the Mongols are helping fuel the violence and are taking it to a more extreme level.

Corrections says the change has also been obvious in prisons.

"What we've seen a lot more of over recent years is this pack attack. Just these randomised attacks on individuals. It's not your one-on-one in a yard somewhere," Beales says.

The threatening of prisoners' family members on the outside also occurs. One report details how a gang coerced a prisoner into carrying meth for them by threatening his mother. He was later caught with the drugs and was charged.

For guards trying to keep control, it takes a heavy personal toll, Corrections' Alan Whitley says.

"They put up with abuse from the gang members, they put up with intimidation from the gang members, staff are assaulted by gang members in prisons," he told Newshub.

The same staff are tasked with recording any recruitments since it's common for non-members to enter prisons and leave as gang members.

But Corrections and sociologist Greg Newbold agree the gang problem isn't just about prison life, it's also societal.

"The more gangs there are, the more attractive they become, particularly in more impoverished areas," he told Newshub.

Corrections is trying to reduce prison gang numbers by gathering more intelligence and offering more support programmes, but working against that is an increasing number of patched gang members entering the system.