The Government's plastic bag ban could cost jobs, says E tū.

Single-use plastic bags will be banned in New Zealand and are to be phased out over the next 12 months, the Government said on Friday.

Retailers rushed to support the ban with many like Countdown, Foodstuffs and The Warehouse already having plans to phase out plastic bags from their shops and supermarkets.

E tū - one of the country's largest private-sector unions, supports the move but its industry co-ordinator Ron Angel told RNZ the transition needed to be fair on workers.

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He believed a few hundred workers were involved in manufacturing plastic bags in this country.

"I think there's enough time for there to be a transition, most manufacturers make a wide range of other products and they will have an opportunity to change their product line and that sort of thing but it is difficult - one day they're making plastic bags for supermarkets and now they're begin told you have to stop within a few months," he told RNZ.

"It's not huge numbers but every job loss is a job loss and it affects that worker, their family and their community."