An artillery shell, thought to be more than 100 years old, has been found in Otago Harbour.

The shell was discovered a few days ago by a worker connected to the dredging of the shipping canal, police said.

The person contacted police, who drafted in the New Zealand Defence Force (NZDF) to assess the bomb.

A controlled explosion was carried out by the NZDF today, police said.

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"The head of the shell is thought to be more than 100 years old, and may have been used in coastal defences," police said.