From 1 July it will be illegal for retailers to hand out lightweight plastic carry bags, including thicker department store bags as well as the biodegradable and compostable options.
These include supermarket carry bags and bags for takeaway food and other retail checkouts.
However, supermarket produce bags, bin liners and dog-poo bags will still be allowed.
Single use plastic bags can take 1000 years to decompose.
While retailers caught still dishing out plastic bags could be fined up to $100,000, the Ministry of the Environment said its preference would be to "take an educational approach and to offer advice to help businesses understand their responsibilities".
It will be up to customers to police the new law, with an online form on the Ministry's website which consumers, retailers and suppliers will be able to use to dob in retailers, which will then be followed up by the Ministry.
Sustainable Business Network chief executive Rachel Brown said despite the lack of direct government oversight on the ban she was confident consumers could keep retailers honest.
"We as people who go into shops are pretty aware of this and actually we're pretty sick of it.
"The awareness of plastics in the oceans has been one of the biggest things, we now know that we're eating plastic in our food so the awareness is really high and people know we have to change."
She said retailers who still had bags left after Monday needed to take responsibility and find a way to recycle them.
Councils in Canterbury and are running a collection scheme to avoid large stockpiles of leftover bags being sent to landfill.
Shoppers on Wellington's Lambton Quay say they would not hesitate to dob in retailers flouting the ban.
Despite the plastic bag in his hand, Joshua White, was still pleased the ban was coming into effect.
"I feel it'll force a lot of people to think about their plastic usage."
Mariana Johnson agreed - her hands were full of shopping bags, but no plastic to be seen.
She was glad retailers were following the lead of supermarkets.
"Either bring your own bag or get a paper one. It's not that hard."
Countdown supermarkets were early to ditch plastic bags, reducing the number going into circulation by 29 million a month.
Retail NZ chief executive Greg Harford said because consumers were so invested in the anti-plastic movement, they would not hesitate to let retailers know if they were breaking the rules.
"One of the interesting things about being a retailer in 2019 is that customers have got a huge ability through social media to call you out and to keep you on track if they think you're doing the wrong."
This article was first published on Radio NZ.