A gang attack on the Waikato Expressway is being seen as rare -- but not unheard of

A motorist is in a critical condition in hospital, after being assaulted by multiple gang members.

Canterbury University's Jarrod Gilbert says while gang attacks on the public are rare, they're more likely when large groups are gathered together.

He says gangs have been growing significantly in size since 2010.

Gilbert says the rising number of young gang members is a concern, because they tend to be more dangerous and anti-social than older gang members.