Gangs are on the rise in Dunedin and Invercargill, police say.

More than 100 patched members and prospects spread across several gangs are now living in the South, police estimate.

However, which gangs they belong to is not being disclosed.

Southern Police District crime services manager Detective Inspector Steve Wood, said the increase was concentrated in Dunedin and Invercargill, reports Otago Daily Times.

In recent years, southern police had observed attempts to establish local chapters connected to established gangs in other centres, rather than the arrival of new gangs, he said. He would not be drawn on the relative size of various gangs in the area.

Asked what measures police were taking to curb gang activity in the South, he said officers were focusing on drugs, known criminals and ill-gotten gains.

"We are targeting ... drug distribution, reducing demand by disrupting supply and targeting known offenders."

"Where possible we are using legislation to restrain and forfeit criminally derived assets."

While new legislation in Australia has resulted in members of outlaw Australian bikie gangs being deported to New Zealand, Det Insp Wood said the number of deportees settling in the Southern district had been low, and police did not believe they had contributed to growing gang numbers.

University of Canterbury sociologist Jarrod Gilbert, author of Patched: The History of Gangs in New Zealand, said the increase in gang members was a national trend.

There had been a marked upswing among the gangs in the last six years.

"It's a quite remarkable turnaround, given for the first decade of the century ... many of them were in a moribund state."

He believed one reason for the rise was the entry of Australian biker gang Rebels in 2011.

"This group formed and found new membership, and suddenly the scene woke up and said 'hang on a second, people are looking to join but they're not looking to join us'," Dr Gilbert said.

"Suddenly they started recruiting and it seemed to snowball."

Dr Gilbert said both outlaw motorcycle gangs and street gangs "have suddenly become cool again."

"The Mongrel Mob, for example, is in particularly rude health.

"We are starting to see new chapters ... something that we haven't seen in many, many years."