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The time would offer customers time to shop in stores which were easy on the eyes and ears by reducing noise, lighting and other distractions.
The low-sensory hour would run between 2.30pm and 3.30pm each Wednesday from October 23 (except for two stores), keeping distractions at a minimum.
A small number of Countdown stores had been trialling the concept after a team member with an autistic child suggested it would be a great idea.
Checkout volumes would be lowered, the in-store radio turned off, trolley collection and shelf-stocking kept to a minimum and no PA announcements - except in emergencies.
Countdown's general manager of corporate affairs, safety and sustainability Kiri Hannifin said they wanted to be inclusive for all Kiwis.
"We know grocery shopping can be an anxiety-inducing experience for some customers and we wanted to help with that," she said.
"By making a few small changes and creating a Quiet Hour, we hope we can make a big difference."
Elderly customers had also enjoyed the stress-free environment of the quiet hour, Hannifin said.
The quiet hour was developed alongside Autism New Zealand, which provided advice to Countdown on how to best support customers.
The invisible nature of autism meant it was hard to create understanding or awareness around the issues people with autism face, chief executive Dane Dougan said.
"We've had amazing feedback from the autistic community, who have benefited from quiet hour over the last year and the increased understanding of autism and sensory needs that it is having as well.
"We're thrilled that Countdown will be offering Quiet Hour in its stores and it highlights how some small changes can create a more inclusive environment that will impact people significantly."
Countdown Silverdale and Countdown Northwest would hold their quiet hour between 9am and 10am, the two exceptions nationwide.
Meanwhile, Auckland City Metro and Cable Car Lane in central Wellington would be the only two stores nationwide not offering a quiet hour.