A loud explosion has been heard at Auckland's Long Bay beach after an unidentified canister washed up.

Fire and Emergency New Zealand's northern shift manager, Carrin Larkin, said Defence Force staff had been called to take a look but firefighters believed it was likely to be a marine marker.

She said the canister was "not unsafe" and one fire truck was at the scene.

A Long Bay resident said they had heard "one big boom" coming from the beach about 9.30am on Wednesday.