The Delta clean and film crew was cleaning a sewer in Murray St when its vacuum pipe became blocked, about 10.15am.
It was a "bit of a shock" when the contractors gave the tube a "good few taps" on the ground and what was believed to be a World War 2 hand grenade rolled out, said the vacuum operator, who did not want to be named.
They recognised the object immediately but thought it was plastic, so picked it up for a closer look.
"It was quite heavy and still had the detonator attached, so we put it back down. And then we called our supervisor."
The street, from Reid Ave to Oban St, remained closed for six hours while the army's bomb disposal squad travelled to Mosgiel from Christchurch.
On arrival about 4pm, the squad verified there was some explosive inside the grenade, but the detonator was missing so it was unlikely the grenade would have exploded, Senior Sergeant Craig Brown said.
"But we have to follow these processes until we are sure."
The squad disposed of the grenade and returned to Christchurch.