A Bay of Plenty sailor thought he'd gone back in time when his catamaran almost struck what appeared to be a World War II-era sea mine off the Auckland coast.
"I thought I was seeing things ... I thought they were preparing to fight World War II again," Allan McDougall said.
The 69-year-old from Whakatane was sailing north from Auckland last month and was "startled to see in the water a large round black metal object floating just at surface level, and bedecked with four spikes", he said.
It was bobbing off the eastern side of Tiritiri Matangi Island, off the coast from the Whangaparaoa Peninsula.
It didn't look weathered enough to be a legitimate war relic, he said, but he was sufficiently concerned to radio the Coastguard anyway.
"I thought it's a mine. It's probably a dummy, but I better call the Coastguard. The police launch came to get it," he said.
He was right. After the object was dragged to the more sheltered western side of the island and examined, he said it was labelled as property of the Royal New Zealand Navy.
"Printed in one centimetre high letters on the black metal was the information that it was, of course, just a training mine. The mine was maybe one metre in diameter.
"I thought maybe they should make them pink or something, so people know it's not a real one."
Sergeant John Saunders of the maritime police confirmed they towed the mine and handed it back to the navy on February 17.
Lieutenant Commander Trevor Leslie, commanding officer of the operational navy dive team, said the training mine had broken its moorings and floated about 500m off course to where Mr McDougall found it. "He stumbled on one of our mines. It's basically a model of one of the old WWII-type mines."
The mine had been laid among others as part of a multi-nation naval training exercise being held "within the Hauraki Gulf and Whangaparaoa Peninsula area".
The "mine counter-measures exercise" formed part of the Western Pacific Naval Symposium, involving 14 countries and more than 600 people. It ran from February 17 and would finish today, the naval officer said.
"Those buoy mines were laid a few weeks ago for this operation. That weekend before, a storm came through and it must have detached from its mooring."
While mine technology has improved since WWII and there are now many different types used by different nations, they do still float away from their moorings and can end up anywhere, he said.
• Four World War II mines were discovered on the seabed during a navy exercise in Auckland's Hauraki Gulf this week. Two were discovered during a routine Autonomous Underwater Vehicle (AUV) operation conducted by the United States Navy as part of the international Mine Countermeasures Exercise hosted by the Royal New Zealand Navy.
The discovery was confirmed by a New Zealand navy dive team.
Two more mines were discovered by a Royal Australian Navy AUV team and later confirmed by Japan's dive team.
The four mines were originally laid as part of a controlled defensive minefield in September 1942.
The Navy advises that due to their age and condition any remaining World War II mines are unlikely to be dangerous.