The national bomb squad has been called to help after a canister thought to contain phosphorus washed ashore on a Bay of Plenty beach.
Emergency services were told about the object at Shelly Bay, at the Bowentown end of Waihi Beach, just after 3pm, police said.
"A canister apparently containing phosphorus has washed ashore on the Coromandel ... the area is cordoned off."
There was no immediate danger to the public, but people were asked to stay away from the scene.
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The New Zealand Defence Force's explosive ordnance disposal team had been notified, police said.
The United States' National Institute for Occupational Safety and Health website says the substance can be used by the military in various types of ammunition, because it spontaneously catches fire in air.
It is also used in fertilizers, food additives, and cleaning compounds.