Armed police threw a cordon around a main road and a primary school in Nelson on Tuesday afternoon after a woman reported hearing three gun shots.
The Armed Offenders Squad was carrying out an exercise nearby and was able to respond almost immediately to the alert in the suburb of Tahunanui. It turned out to be fireworks.
Senior Sergeant John Price said it was highly irresponsible behaviour.
"Setting off fireworks any time outside of the traditional Guy Fawke's period is pretty stupid, in my view. They are dangerous and they cause alarm for other people in the community, which is not what we want to happen. We want people to feel safe and be safe in the community."
The cordon around the school has been lifted and Tahunanui Drive is open again.