Victoria retailers that haven't yet ditched flimsy, lightweight plastic bags voluntarily will soon find themselves having to do so as a matter of law.

Woolworths ditched single-use plastic bags last week and Coles is set to follow suit next month.  For the rest, the Andrews government is expected to introduce legislation next year that will ban lightweight plastic bags across the state.

"While we would like to eventually eliminate use of all plastic bags in Victoria, we want to be sure we have appropriate alternatives in place first," read a discussion paper released on Wednesday and quoted by Fairfax Media.  "We do not yet have clear alternatives that will prevent food contamination or properly contain hazardous medical waste."

The legislation will target all plastic shopping bags less than 35 microns in thickness, such as those commonly used at supermarket checkouts. 

"We are absolutely committed to doing more to reduce plastic pollution in our environment," said VIC Environment Minister Lily D'Ambrosio.