Police say an unexploded shell discovered under a Wellington house two days ago and only reported to them on Friday morning could have exploded at any time.
The artillery shell was discovered by a resident who had been digging up ground to build a deck at the property on Boyd Terrace in the suburb of Newtown.
About 50 houses, including part of a retirement home, had to be evacuated on Friday while the Defence Force took the shell to nearby MacAlister Park where it was detonated.
Senior Sergeant Hamish Milne said the shell, believed to be 150 years old and the size of a two-litre bottle, was live and too dangerous to be taken to a more remote location to be destroyed.
Mr Milne is advising people in the area, the site of a former military barracks, to check their properties for similar explosives and to call police immediately if they find anything.