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The incident happened near the Gables Arcade on High St in Rangiora.
Six people were treated at the scene and three people have been transported to Christchurch Hospital - one with moderate and two with minor injuries.
Fire and Emergency have twice visited the area - after reports of a strange smell.
They cleared all businesses from the building and sent in specialists with gas detectors to find the source.
Police helped move people away from the vicinity of the Gables Arcade, a police media spokesperson said.
Among the stores evacuated was that Spark store on Rangiora's High St.
A Spark spokeswoman said "some of our team members who felt unwell are seeking medical advice and our store will remain closed until the area is deemed safe for our people and our customers by the relevant authorities".
A St John media spokesperson said they were called to the incident just after midday.
Fire and Emergency NZ shift communications manager Andrew Norris said a fire crew had been out to the store earlier in the day after being called about a strange smell in the building, but couldn't locate anything.
After being told by St John this afternoon they were treating more patients, a fire crew has returned and has been through the building, including checking the ceiling cavity for the source of the smell. A specialist unit from Christchurch was in attendance, using a gas detection unit to try and identify the source.
Norris said the only possible source at this stage was some old batteries in the building.