Sarah Hird is such a greenie that she even takes her own mug when she goes out for coffee.

The Whanganui woman made a special trip to the Countdown supermarket on Thursday.

She took her own grocery bag so that she could go in the draw for a prize from three Whanganui High School girls.

Zara Roestenburg, Holly Parkes and Ashley Fowler are working toward NCEA credits in Social Studies by highlighting an issue to change a policy.

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They want supermarkets to stop offering plastic bags when people buy groceries.

They planned to be outside that Countdown supermarket on Thursday afternoon, taking the names and phone numbers of people bringing their own grocery bags.

Those people would go in the draw for a free coffee, $20 voucher and supply of reusable bags.

"I try to save petrol by putting things together, and I usually bring my own shopping bag," Hird said.