An injured man was taken to hospital by ambulance in a serious condition following the incident.
One shopper, who did not want to be named, told Hawke's Bay Today it was pretty quiet in the supermarket prior to the incident unfolding.
He said he then heard "repeated banging like a trolley running into one of the windows and yelling".
He said a small group of shoppers stayed at the back of the store while the incident was happening.
He said after the incident a few shoppers ventured to the front of the store.
"A man was being attended to by the checkout staff giving first aid," he said.
"The injured man was sitting in a chair and had a couple of Countdown staff attending to a head wound or two. There were blood drops along the floor."
He said customers were kept inside for a short period as a precaution.
"We were kept inside temporarily for safety in case the [alleged] offender was still nearby until being let out.
"Customers arriving were diverted to the other Countdown across the road."
He said police were on scene quickly and the supermarket staff did a fantastic job managing the situation.
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