NEW laws banning the sale of knives to minors in Victoria are expected to be introduced within weeks but retailers say the state government has bungled the rollout, leaving them struggling to cope with their new responsibilities.

The reform - which extends to stopping the sale of plastic knives to under 18s - follows a spike in knife crime, with robberies at knifepoint soaring almost 50 per cent in Victoria during the past five years. Possession of knives and other weapons was up 25 per cent in the same period.

The state government flagged the bans in March when it also announced police would get new powers to fine people carrying knives $1000 on the spot and wield broader search powers.

Thousands of retailers who sell knives will be affected by the new laws, under which any minor caught trying to buy a knife will face an on-the-spot fine of $234, or $1400 if the matter is serious and taken to court.

Anyone caught selling to a minor could be fined more than $2300.