UPDATED 3.53PM: New Zealanders use 1.6 billion plastic bags every year, but the Green Party has launched a plan to bring that number down.

It wants to make it mandatory for supermarkets to charge 15 cents per bag at the checkout.

LISTEN ABOVE: Kerre and Mark spoke with Jamie McDell about Boomerang Bags - an organisation working to reduce the use of plastic bags

Green Party waste spokesperson Denise Roche said many supermarkets support the move.

"A lot of shops say they'd love to charge for plastic bags, but they won't because nobody else is."

"Their margins are so tight that they will compete on every angle," she added.

Roche said those who don't want to pay have plenty of other options.

"The thing is that you don't have to buy the bag, you can bring your own, you can grab a box."

Massey University environmental anthropology lecturer Trisia Farrelly said it's estimated that by 2050, there'll be the same volume of plastic in the ocean, as there is fish.

"If we're ingesting fish, it's really bad for public health and it's not at all good for the sustainable and healthy volume of our fish for the future."

The policy is similar to a move in the UK, which the Greens say has reduced plastic bag use by 85 percent.