Where does bag dosh go?

"We’ve found that the public is far more accepting of a charge when that money is reinvested into the charitable sector." Waste MINZ chief executive Paul Evans.

No surprises here: most people will stop using single-use plastic bags if they have to pay for them.

We so begrudge handing over that 10 extra cents, The Warehouse dropped the number of bags it issued by 67 per cent since it began charging in 2009.

Of course, many other shops in New Zealand still give out bags for free, while some have announced plans to ban them altogether.

But when you do end up forking out for a bag, do you know what happens to that money? And does it make any difference to you as a shopper?

The Warehouse invests its 10 cents per bag in community projects local to stores - from Garden to Table projects in primary schools to "zoofari" programmes helping children from low decile schools visit Auckland and Wellington zoos to learn about the environment and conservation.

More than $3.8 million has been raised through The Warehouse’s Bags for Good initiative since 2009, according to the retailer.

Pak ’n Save charges 10 cents per bag in the North Island, and 5 cents in the South Island. It says the money goes into "keeping prices as low as possible". So presumably, back into the company.

New World will introduce a 10 cent voluntary donation per plastic bag on February 1 next year, with the money going to environmental causes such as Sea Cleaners, before it phases bags out completely. The company has pledged to be bag-free by 2019.

Countdown was charging 15 cents each for compostable bags at its "plastic bag-free" supermarket on Waiheke Island (but not at most stores). It has announced plans to phase out single-use bags by the end of 2018, and said it would drop the charges for its reusable bags from $1.39 to $1 to help encourage shoppers. Countdown says it invests $8 million annually in community and environmental initiatives.

Kiwis are generally more supportive of charges for carrier bags if we know the money is going to a good cause.

A Waste Management Institute of New Zealand (Waste-MINZ) study in 2017, which surveyed 1000 people, suggested 40 per cent of Kiwis were against charging for plastic bags, or were neutral. But if the money was donated to charities, many of those people changed their minds and said they would support charges.

Overall, that meant that roughly two-thirds of New Zealanders supported a levy if the money went to charity, and 43 per cent were happy to pay 10 cents or more.

"When people weren’t in favour of charges... generally they said that it might just go back to government and into their coffers or that retailers might actually just use it to increase their profitability," said Waste-MINZ chief executive Paul Evans.

"We’ve found that the public is far more accepting of a charge when that money is reinvested into the charitable sector."

Evans would like to see a similar system to the one in England set up for big New Zealand retailers. That system sees retailers required by law to charge 5 pence (9 NZ cents) per bag, with the money mostly donated to a charity of the business’ choosing.

The number of bags issued fell by more than 70 per cent within a year after the levy was brought in. But England’s biggest grocer, Tesco, came under fire for taking more than 10 per cent of the proceeds from selling carrier bags to cover administration costs.

Trisia Farrelly, senior lecturer at the school of people, environment and planning at Massey University, prefers a system like Ireland’s, where levy money is put into an environment fund which finances waste management, litter and other schemes. Some 90 per cent of consumers in Ireland were using long-life bags within a year of its introduction in 2002.

Here, the government hasn’t shown any sign of bringing in mandatory charges for plastic bags. But if it did, Farrelly said the money should "go to an environmental fund that helps us to move towards a direction of phasing out plastic bags into a complete ban over the next few years".

The funds could be used to support "some green chemistry and some good research and development to produce alternative, environmentally benign plastic or plastics".

Anything less than a 15 cents-per-bag charge would have little impact on Kiwis’ behaviour, she added.

Farrelly also said it was important that decisions were made at a national level, rather than being left down to shops. "If it is governed purely by individual shopkeepers you’re going to create an uneven playing field whereby one business could charge less... then you’ve got a competitive field and that doesn’t work."