After a month dwelling on the problem of excess plastic, Dierdra McMenamin has had an audacious idea.

The main organiser of Plastic Free July is challenging Whanganui supermarkets to give away good quality, reusable bags for a whole month.

"Let them show they're good-hearted and do care about people and the planet, and take action. They can lead the way as big businesses with the financial capability," she said.

She's suggesting each supermarket places a box full of long-lasting reusable shopping bags outside its entrance.

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People who have reusable bags they don't need can add to the collection.

People entering supermarkets without their own bags would pick up enough to take their groceries home in.

A month would be long enough for people to get into the habit of bringing their own bags, she said.

Listening to Dr Trisia Farrelly's talk in Whanganui on July 20 has made McMenamin very concerned.

Farrelly said only 9 per cent of plastic is recycled, and plastic that ends up in the sea degrades into smaller and smaller particles, and gets into food and water.

"We are literally now eating plastic," McMenamin said.

To her, single-use plastic shopping bags are just a symbol of a much bigger problem. Phasing them out could get people to think about how to manage with less plastic.

Yesterday the Village Snob coffee cart was offering coffee at half price to people who brought reusable cups. The offer was part of Plastic Free July, because takeaway coffee cups have plastic lids.

Owner Paul Harris said as July wore on, more and more people got the plastic free message and came with their own reusable cups.

"I've been actively pushing it and I think it's good."