On a clear, full-moon night off the North Canterbury coast, New Zealand navy sailors spotted what they thought was the periscope of an enemy submarine.
It was 75 years ago this month and New Zealand was in the second-to-last year of World War II.
The minesweeper Wakakura chased the sighting from a range of 200m, the supposed submarine dived and the Kiwis attacked, dropping three depth charges. The third caused a more violent explosion than the first two.
Small oil slicks seen on the surface were assumed to be residue from the depth charges, historian Peter Dennerly, formerly of the National Museum of the Royal New Zealand Navy, has written.
Sixty years after the attack, a talkback radio caller who identified himself as a recreational diver claimed his club had found the hull of a Japanese submarine when diving off the Canterbury coast.
Former Radio Pacific talkback host Allan Dick said he remembered taking the 2004 call, but not who the caller was and the tape recordings were kept for only three months.
"I had started that day talking about the landing by the Japanese on the coast of Ashburton in about 1944," he told the Herald on Sunday.
"They held up a grocery delivery van … the only witness was the delivery van driver. That's quite a widely known story in Ashburton; it's probably an urban myth."
The North Canterbury Dive Club's president Warren Price said there had been a lot of talk on the question of a Japanese submarine in Pegasus Bay, but no club members had come across one.
But several enemy submarines did cruise the New Zealand coast during the war.
German submarine U-862 even entered Gisborne's port, in January 1945, but did not sink any ships.
Earlier, in May 1942, was the visit of Japanese submarine I-21, the second of three Japanese submarines confirmed to have come to New Zealand. It moved on to Australia where its commander oversaw the attack at Sydney in which a ship in the harbour was sunk by a torpedo, with the death of 21 men.
Japanese seaplanes, carried in submarines, flew over Auckland twice in 1942 and Wellington once.
In February and March 1944 there were six possible submarine sightings, culminating in the Wakakura attack off Motunau on March 8.
In one of the sightings, crew on the inter-island ferry Rangatira reported seeing the track of a torpedo passing close across the ship's stern when it was 19km off Kaikoura.
Christchurch Press readers were told ferry sailings were cancelled in March 1944, but not why. Source / National Library
It may have been a porpoise, but ferry sailings were cancelled and east coast South Island ports were closed for departures for several days while ships and planes hunted the submarine.
The episode, however, remains a mystery.
Dennerly concluded that a submarine was in New Zealand waters in early 1944. It was "possible, albeit unlikely" that it was sunk by the Wakakura.
Navy museum researcher Michael Wynd is more sceptical, based on Japanese naval records and the contraction of the area in which Japanese submarines could operate after 1943.
"There is no evidence that Japanese submarines were operating off Christchurch in March of 1944."