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Prime Minister Jacinda Ardern and Associate Environment Minister Eugenie Sage made the commitment at Wellington's Lyall Bay beach this morning.
This year 65,000 Kiwis signed a petition calling for an outright ban, and Ms Ardern said they were listening to those demands.
"Every year in New Zealand we use hundreds of millions of single-use plastic bags - a mountain of bags."
New Zealand is one of the highest producers of urban waste in the developed world, per capita, according to OECD data.
Ms Ardern said many plastic bags end up polluting the country's coastal and marine environments and cause serious harm to all kinds of marine life.
"And all of this when there are viable alternatives for consumers and business. It's great that many people are already changing the way they shop but it's important we take the time now to get this right."
Ms Sage wanted the public to give feedback on the best ways for this ban to be phased in - and has opened consultation until 14 September.
She wanted feedback on all aspects of the change, including options for the date the phase-out is to be complete by, what bags should be included, any retailers that should be exempted, and how best to help people with the transition.
"Public calls for action have encouraged a significant number of retailers, including supermarkets, to move on single-use plastic bags. We want to support their efforts by ensuring the retail industry moves together in a fair and effective way," she said.
Ms Sage said many countries and major cities around the world have successfully taken action on plastic pollution in recent years, and she was confident New Zealanders could also embrace the change.
Scientists also estimate that eight million tonnes of plastic enter the world's oceans every year, and if nothing changed, there could be more plastic in our oceans (by weight) than fish by the year 2050.
Recently 13 local and multi-national companies committed to use 100% reusable, recyclable or compostable packaging in their New Zealand operations by 2025 or earlier.
What you need to know about the mandatory phase-out plans:
Which bags does this ban include?
'Single-use plastic shopping bag' refers to those with handles commonly found at supermarket and other retail checkouts, and potentially thicker bags provided by some retailers.
As part of the consultation process, the government is looking into the maximum level of thickness that should be allowed.
The consultation document has more information on the different types of bags being considered.
Are biodegradable and compostable bags included in this consultation?
The proposed mandatory phase-out includes "biodegradable", "oxo-degradable" and "compostable" bags - all types of degradable plastics.
The government is recommending reusable bags over single-use alternatives for now because New Zealand does not yet have the nationwide infrastructure established to ensure that biodegradable and compostable plastics are processed in the right way.
Biodegradable and compostable plastics can be as harmful to nature as their non-biodegradable counterparts if they do not enter an environment that they are designed to break down in. Oxo-degradable bags break down into microplastics, rather than nutrients and materials which nature can absorb.
Why are you only consulting on a mandatory phase-out - why not a levy?
The Government believes a mandatory phase-out is more effective and of less cost to the public as it can be done under existing legislation through regulations under the Waste Minimisation Act, whereas a levy or mandated charge would require new legislation.
It also said introducing a legislated levy or mandated charge over the top of supermarkets taking recent action to ban plastic bags would be inconsistent and create confusion.
How will I get my shopping home if I forget my bags?
The Government is proposing to give people a long lead in so that the public has time to adjust to reusable bags along with retailers who have yet to make the change.
It also recommends customers take advantage of the offer by some supermarkets to use low-cost reusable bags or free used cardboard boxes. Or to bring their own wheeled trolley bags, backpacks or home-made bags.
Retailers are also being told to consider alternatives to single-use plastic bags, including reusable bags in heavier-duty plastic, composite bags of hessian with other materials, and long-lasting bags made of lightweight nylon, cotton, recycled fabric or jute.