A gang expert says several factors are contributing to the rise in violence on our streets.

Police are investigating a serious assault by a convoy of gang members on motorbikes on the Waikato Expressway that's left a motorist critically injured.

Canterbury University's Jarrod Gilbert says gangs have been growing consistently since 2010.

He told Mike Hosking many of the new members are younger people who are determined to make a reputation for themselves.

“That increases the chances of these types of issues. When you get a group of people like that together at once, often they will behave in ways they might not individually.”

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