Firefighters with breathing apparatus converged on a premises in Glasgow St after a report of an explosion.
There were no indications of a fire, but a Fire and Emergency New Zealand (Fenz) spokesman said a possible chemical spill needed to be investigated.
In an update around 8.50pm, a Fenz spokesman confirmed there had been a spill, but there was no word on what the chemical was.
Sand had been used to absorb it and prevent it from spreading.
Six fire crews attended.