Prosecuting drug dealers for the crime of selling ice to customers who go on to commit murder would be unenforceable, warns critics of Matthew Guy's latest "tough on crime" policy.

If elected in 2018, the Coalition has promised to bring in a mandatory 10-year minimum sentence for dealers who provide ice or other drugs to someone who then carries out serious violent crimes, including "mass casualty attacks" and murder.

Opposition Leader Matthew Guy's latest crime policy has been criticised as impractical. Credit:AAP

But Sentencing Advisory Council chair Arie Freiberg cast doubt on the viability of the plan, saying the proposed offence would face a "number of issues of proof".

Emeritus Professor Freiberg said proving that a dealer should have known one of their customers would go on to commit the crimes would be a major hurdle.