The Green Party wants the Government to make it mandatory for supermarkets to charge for plastic bags.
Foodstuffs, which operates New World and Pak 'n Save, starts charging a fee of 5 cents per bag from Monday.
The Warehouse started charging 10c for shopping bags in April.
Green Party co-leader Russel Norman says the companies are trying to do their bit for the environment and the Government needs to ensure others do the same.
Dr Norman says he's worried companies which charge for bags will be undermined by others which don't.
