A criminologist says taking a hard-line approach to gangs on income, drugs and violence won't solve those problems. 

The National Party's discussion document includes a proposal for gang members wanting a benefit to prove they don't have illegal income. 

University of Canterbury Criminologist Greg Newbold told Simon and Phil that will always be hard to prove, as is the case when it comes to the perception gangs are the main peddlers of drugs. 

LISTEN ABOVE