On February 12 1962, de Havilland DH90A Dragonfly ZK-AFB "Kiwi Rover" went missing on a scenic flight from Christchurch to Milford Sound.
The search for the aircraft, its pilot Brian Chadwick and four passengers - Louis Rowan, Darrell Shield plus honeymooners Elwyn and Valerie Saville - was headline news on both sides of the Tasman, as two of the passengers were from Australia.
In spite of the largest aerial search undertaken in New Zealand, and one of the most extensive in the South Pacific, nothing has ever been found.
On the eve of the 55th anniversary of the plane seemingly vanishing into thin air, Kiwi film-maker Bobby Reeve and his family have launched an extensive new search.
Reeve is convinced a heeled fashion boot he found in a mountain stream in the Hopkins Valley, near Lake Ohau, during his last search two years ago is proof he is on the right track.
"As soon as we saw it, we knew it was out of place," Reeve says. "It was more than four hours' walk from civilisation, so we know we are getting close to the Dragonfly.
"It is only a matter of time before it is found."
Reeve, his wife and two sons spent up to six weeks at a time living in the remote Hopkins Valley, grid-searching the dense bush for wreckage.
On the day the plane went missing, there were 18 reported sightings to both the east and west of the mountains.
"We have had the boot analysed and it dates from the late 1950s or early 1960s," Reeve says. "The year before we found it there was a huge flood in the area and I suspect the boot was dislodged by the water and shingle.
"I contacted a sister of Valerie Saville, the Kiwi woman who vanished with the plane, and she said it sounded just like the kind of thing Valerie would have worn and it seemed about the right size.
"If we hadn't found the boot we wouldn't be going back in there, we are convinced we are getting close."
Aviator Richard Waugh, whose father was good friends with the pilot, documented the flight and the mystery surrounding it in a book, Lost Without Trace: Brian Chadwick & the Missing Dragonfly.
Mr Waugh, who is also an aviation industry chaplain, says he will be watching the latest search with interest.
He hopes a renewed focus on the 55th anniversary of the aircraft's disappearance could lead to a member of the public remembering something significant and coming forward.
"The find of the boot was interesting and it will be even more interesting if anything else unusual is found in that area," he says. "I'd dearly love to say a prayer at the crash site and help organise funerals for those who died."
New bride Mrs Saville was just 22 when she disappeared. She was the second youngest in a family of 12 children growing up near Gisborne.
She had been an office worker in Sydney before marrying Elwyn Saville, who was 20 when he disappeared.
One of her sisters, Joyce McGarva, of Gisborne, described her younger sibling as a "sweet little piece" who liked to make everyone happy.
The other Australian on the flight, 25-year-old Mr Rowan, was a cabinetmaker who worked in Papua New Guinea before coming to New Zealand for a holiday ahead of his planned return home to Sydney.
"It would nice to bring some proper closure for the families after all this time," Waugh says. "There were no funerals held in 1962 and the families have been waiting and waiting but nothing has happened.
"The pain still cuts deep for them.
"Every other month I still get a call or an email with information from someone or from family asking if there have been any developments. This is a mystery that won't go away."
Waugh believed deteriorating weather had caused carburettor icing - but, until the plane is found, no one would ever know what happened that fateful day.
As the official accident report put it: "The aircraft disappeared in the course of a flight from Christchurch to Milford Sound but the reason for its failure to arrive at its destination has not been determined."
Since the disappearance of the Dragonfly, there have been four other fixed wing aircraft lost without trace in the same southern region of the South Island.
In total, including those aboard ZK-AFB, 21 people - five pilots and 16 passengers - have vanished.
"The area is a bit like New Zealand's Bermuda Triangle," Waugh says. "It is Tiger country out there, with very rugged terrain and tough weather.
"But someone might hold a clue, have passed a piece of wreckage or tripped over it and took it home, and it is still sitting in their garage.
"My other great hope is that advances in technology and metal detecting techniques will eventually unearth the plane."
While Mr Reeve and his family scour the area where they found the boot, another group of searchers are still convinced the plane came down on the West Coast.
About eight people, led by Gavin Grimmer, head up the Jacobs River near Fox Glacier for a few days each February looking for clues.
They get dropped in by helicopter, as it would take half a day to walk in through the bush.
Grimmer is not convinced the boot found to the east came from the plane.
"The aircraft was last seen at the Jacobs River area," he says. "I believe the pilot flew in there and got out unseen and I think the plane came down in another location nearby.
"I researched this full time for 18 months and my theory is the plane was caught in freak weather and was caught in an enormous updraft.
"I now think Brian Chadwick got the plane out of the valley and was heading for Queenstown before it came down."
The story is almost like a fairy tale but these were real people and there are loved ones out there who still desperately want to know what happened.
Grimmer first became hooked on the mystery in 2006 and reckons his searches would have cost a professional team in the region of $500,000.
"It costs a lot of cash to do this and I've paid a lot of it from my own pocket," he says. "I first got interested in looking for the plane when Google Earth came along, and that got me started.
"I will keep going as long as I am able. Even if the aircraft is in a lake, it will eventually be found."
Grimmer insists he is past the point where it just the mystery which keeps him motivated.
"Family members have contacted me down the years, thanking me for my efforts. There are a lot of people still hurting, and they want answers.
"The story is almost like a fairy tale but these were real people and there are loved ones out there who still desperately want to know what happened."