Apparently nothing of concern was found by emergency services.
Naylor Love workers were evacuated from the shopping mall on the corner of Camp and Beach Sts about 1.30pm and a busy street at the heart of the town remained closed for most of the afternoon.
Fire and Emergency New Zealand shift manager Andrew Norris said three crews had responded to reports of a small fire in the main switchboard at the site that was "quickly extinguished or out on arrival".
However, a spokesman for Fenz later said there was a suspected chemical reaction between two substances.
He said a fire investigator was inspecting the area and the streets remained closed while checks were carried out for gas.
A Fenz spokeswoman said no gas was found, and affected streets had apparently reopened before 5pm.
An official "stop" message was filed by three firefighting crews in the area at 4.45pm.
A police spokeswoman said the operation had been run by Fenz and police provided routine traffic control support.
Police had remained in a supporting role, and it was not immediately clear exactly when the specific streets involved were reopened.
Nothing suspicious had been reported to police, or required further investigation, and the police role had ended as firefighters completed checking the area and left, she said.
Businesses in Beach St were earlier forced to shut up shop as Fenz enforced a cordon.
The Skyline Enterprises owned-mall is undergoing a $26million redevelopment that started earlier this year and was expected to continue for 18 months.
Naylor Love did not provide comment.
