Half the plastic bags we use come from supermarkets.

The useful life of a plastic bag is a brief one, and it's followed by decades of hanging around.

Most of our single-use carriers are used for about 12 minutes, but they could take a century to break down.

In the meantime, they are clogging up landfills, polluting our waterways, or blowing into the bush.

BIRTH

Most plastic bags begin their lives in Asia in China, Indonesia or Malaysia, where ethylene from natural gas and petroleum are used to make polyethylene.

A standard shopping bag is created by melting the polyethylene plastic, and blowing air to create bubbles, which forms the film bags are cut from. Waste material generated during the production of the classic high-density polyethylene (HDPE) shopping bag is usually recycled by the manufacturer.

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According to research commissioned by the UK government, the energy for manufacturing is mainly met by grid electricity. In China this is mainly produced from coal, but other bag-producing countries, such as Malaysia, have a lower reliance on coal.

To create enough film for 1000 bags requires roughly 6.151 Kilowatt hours of electricity - that's equivalent to a running an oven for three hours.

Shipping bags to New Zealand and on-road transport within the country also contribute to the environmental impact. For bags produced in China, the average bag will travel roughly 11,000km.

LIFE

Research from charity and industry groups suggests Kiwis take home somewhere in the realm of 700 million supermarket plastic bags every year.

That doesn't even take into account all the other bags used to carry home takeaways, clothing purchases, or that new piece of tech. In fact, supermarkets are only estimated to contribute just over half of all plastic bags used.

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One piece of research submitted to Victoria University in 2007 estimated total bag usage to be 1.29 billion bags. That's five per week for every person in New Zealand and the figure will only have gone up since.

It's thought each plastic bag is in use for only about 12 minutes before we bin it, drop it, recycle it, or stuff it in our plastic bag drawer for re-use.

DEATH

In 2015, the then-National government rejected calls for a mandatory charge on all plastic bags, in favour of funding the Soft Plastic Recycling Scheme. This scheme takes all soft plastics - anything you can crumple up in your hand.

The whole lot is shipped to Australia, where it is melted down and mixed with scrap and recycled hard plastics to make items such as decking, fenceposts, and park benches.

Run by the Packaging Forum, the scheme now covers much of New Zealand. In 2017 up to September, the scheme collected 240 tonnes of soft plastic bags (around 60 million individual bags). There are plans to expand across the central North Island within the next two years - and Stuff has become a sponsor to help make this happen, giving our subscribers improved access to recycling wherever they are.

But that's only a few per cent of the soft plastic waste being sent to landfill, and statistics generated by industry advocacy group the Packaging Forum suggest plastic bags accounted for 1.5 per cent of litter.

But these figures have been criticised by environmental groups, who say they massively downplay the environmental impact of plastic bags. The litter survey counted all items bigger than a bottle cap and divided them into categories, meaning percentages of overall waste were based on numbers, rather than size, or visual impact.

Research from the New Zealand Packaging Council from 2009, estimated bags made up 0.2 per cent of the overall waste stream - but those measurements were based on weight. The relative impact of shopping bags would be far higher if the study looked at volume, some critics claimed.

Sustainable Coastlines, an organisation focused on marine protection, says it has collected over 1.2 million litres of rubbish over eight years. CEO Sam Judd estimated that 77 per cent of marine waste came from single-use plastics, with plastic bags the fourth-biggest contributor.

WHAT'S NEW ZEALAND DOING?

​In May, the mayors of Wellington, Auckland and Dunedin reignited the issue by calling on the Government to impose a levy on bags - or step aside, and allow local government to do so.

Retail NZ, whose members account for about two-thirds of total retail turnover in New Zealand - and environmental groups showed their support, and the open letter has now been signed by 96 per cent of all district and city mayors in the country.

Levies have been imposed in several countries across the world, such as the UK, Ireland, and Denmark, with mandatory charges commonly leading to drops in bag usage of between 66 and 90 per cent.

However, the previous Government maintained a firm stance that industry-led change is the way forward. Since then, Countdown and New World have announced they will phase out bags in their stores next year, and other stores are now doing the same.

A working group was set up in June, which included the Packaging Forum - an packaging industry advocacy body - and New Zealand's largest supermarket chains.

Since its creation, the group has met once for one "introductory meeting". There is still no date set for a second meeting, any stated objectives, or deadline for action.

The recently-minted new Associate Minister for the Environment Nanaia Mahuta said Labour would consider a levy, based on the UK's model, but no policies had yet been put in place.

"We recognise the strong support for such a measure by the local government sector, and the damaging impact that single use plastic bags have on our environment," Mahuta said.