A Kiwi mum who continually relives the moment her son was killed in a crash by a drug-affected driver says she cannot support legalising cannabis.

"Ask any parent who has lost a child to a cannabis-impaired driver if they would agree to it being legalised," Barbara Semb said in a recent letter to the editor published in the Herald.

Her son, Chris Semb, died aged 51 in Queensland, Australia, in 2014 when an oncoming van crossed the centreline near Bundaberg and hit his motorbike.

He was thrown up to 60m down the road by the force of the collision.