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Shops near The Hour Glass - a high-end jewellery store on Queen Street - went into immediate lockdown during the incident about 3.45pm.
Police said six people used weapons to get into the store and stole two bags of jewellery.
Bystanders and an off-duty officer stepped in and recovered the two bags.
But the officer was hit in the head by one of the robbers and taken to hospital with moderate injuries.
One male was arrested at the scene. Police are hunting for anybody else who was involved.
The jeweller is still working to determine if all items stolen have been accounted for.
RNZ's Susie Fergusson was in a nearby shop and said staff moved quickly to protect customers.
"People were moved further into the store and they pulled the shutters down. Certainly people were frightened in the make-up store I was in."
Another witness said there was an altercation and shouting, and road cones had been thrown.
The jewellery shop has been cordoned off as investigations take place.
At least five police cars and an ambulance responded to the incident.