The murky world of private investigators has intrigued for decades.

Private eyes have even inspired their own genre of hard-boiled fiction with legendary tough-guy characters like Chandler's Philip Marlowe, Hammett's Sam Spade, and Spillane's Mike Hammer. Hired to crack the toughest whodunits, they operate on the fringes of society, and sometimes, the law.

A top Australia private investigator has been enlisted by the desperate mother of 6-year-old Que Langdon, who was last seen on a catamaran leaving Waikato's Kawhia Harbour on December 17 with her 49-year-old father, experienced sailor Alan Langdon.

Col Chapman, a child recovery specialist, arrives in New Zealand today to start sniffing around for clues as to their whereabouts.

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His firm, Child Recovery Australia claims to have successfully recovered more than 120 "parentally abducted" children in the past 26 years, from all around the world, including Brazil, Cambodia, China, Egypt, England, Israel, Lebanon, USA, and Zimbabwe.

Chapman has been brought in to find Que by Ariane Wyler who hasn't seen her daughter for 20 months.

Chapman has said he thinks Langdon and the girl are already well on their way to Australia. Information garnered from seasoned professional sailors, suggests that their tiny, 6-metre catamaran could already be in Australian waters.

New Zealand police also admit the possibility of what it calls a "foolhardy" trans-Tasman runner and today called off its massive search by sea and air. Border alerts are in place and Interpol is involved.

Tracking the pair down is the hard part. Especially if they do make the east coast of the vast continent, where Chapman says the pair could easily hide.

But if Chapman does discover where they end up, reuniting Que with her distraught mother Wyler would be the easy part, he says.

"This one is very difficult at the moment, because we've got to locate them first. Once we locate them, recovery doesn't concern me. It will be done, I have no doubt about that," Chapman says.

Private investigating, especially the role of child recovery agents, is a strange, underground world.

It hit global headlines earlier this year after Brisbane mum Sally Faulkner was jailed in Lebanon with a 60 Minutes television crew after a failed attempt to grab her children off a Beirut street.

Chapman, who bid for the recovery job but lost out to rival child recovery outfit, Child Abduction Recovery International (CARI), run by Adam Whittington, was openly critical of the botched operation.

Whittington and Chapman have gone back and forth accusing each other of being cowboys, mercenaries or even con men.

In 2015, Whittington slammed Chapman for his role in helping former Neighbours star Eliza Szonert allegedly snatch her son while he was at a restaurant with his father in Malaysia.

Chapman accepts his firm has "made lots of mistakes" over the years, but says he always plays within the law.

He admits that sometimes his tactics to retrieve the children (generally to comply with court orders) are inventive. Like the time he was employed to recover a child living with his mother in a small Malaysian village. Chapman rented a house, got friendly with the locals, and hired the mother as his housekeeper.

"The child was on a plane before she even woke up the next morning," he told the Sydney Morning Herald.

Ariane Wyler and six-year old daughter Que Langdon, who has been missing with father Alan Langdon on a catamaran since December 17. Photo / Supplied
Ariane Wyler and six-year old daughter Que Langdon, who has been missing with father Alan Langdon on a catamaran since December 17. Photo / Supplied

People always question the techniques, Chapman says.

Although each country interprets family law differently, sometimes even between provinces, Chapman is proud to claim a record of having had no agents arrested or jailed yet.

He also gets steady work from New Zealand parents, including two cases in the past three months, with children being taken to Croatia and Taiwan.

"We've had kids taken to tin shacks in Cavite in the Philippines with no hot water, electricity and a piece of fruit every day, from London townhouses. They're quite often changing their names, hiding, changing their appearances. Everyone can find fault with some method [of recovery]," he says.

"People are very interested [in the profession]. Some people are negative to it, it's just about informing them more, and saying 'This is how it's done,'. People think we're just kidnapping someone. But ninety per cent of our cases go back to the family courts."

The physical recovery of the children is always filmed and non-violent. It is often done in public places such as parks or restaurants.

"Imagine if you're walking with your 6-year old child, you go to Maccas [McDonald's], you go to the bathroom, the mother walks up from behind, it's not like one of us walks up from behind, and says, 'Hey darling, we're going'. And they do. It's always some situation like that," says Chapman.

"The kids aren't screaming or crying. We do passive recoveries, it's not aggressive. Whilst it's not always pleasant, it's certainly a necessity."

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Wyler first employed Chapman last year to find Que.

The Langdon family of three, Langdon, Que and Wyler, had been a complete unit and living in Port Vila, Vanuatu, when Cyclone Pam struck in March 2015.

The category 5 severe tropical storm battered the island, claiming up to 16 lives and sinking at least 20 boats. It sank their 46-foot catamaran, Sanyasin, and the family was lucky to escape with their lives.

After the cyclone, when the Australian and New Zealand air forces were repatriating people, Langdon took off for Australia with Que, Chapman says.

Chapman was enlisted by Wyler to find Langdon, who kept moving about.

He says he eventually tracked Landgon to the New South Wales town of Nimbin where Langdon and Que were living in a campervan on a large farm.

"We suggested to Ariane to do her recovery but she didn't want to ... She's still doing the same thing. But I'm saying to her, the system doesn't always work, the system isn't always right. It's frustrating."

Chapman says he believes Langdon and Que are bound for Australia in the white wooden Tiki catamaran, which has blue anti-fouling paint under the waterline, is modified with a Bermuda mast, and a sail ID number of T878.

His original sleuthing suggested it would take Langdon between 30 and 40 days to cross the Tasman in the small vessel.

But latest calculations suggest the catamaran could be capable of much quicker progress, especially if Langdon had rigged it with a self-steering auto-pilot.

"This catamaran has turned out to be quite a remarkable boat," Chapman said.

"It's extremely reliable, it handles brilliantly apparently, better than any other 6m catamaran. It depends on the weather every day and whether he is sailing hands on, or by auto-pilot ... but it all contributes to the fact he could be here now.

"It's a big trip but once he crosses a line a couple of hundred kilometres offshore [of Australia], he looks like a coastal cruiser. He doesn't look like he's come from New Zealand in that little thing."

Chapman will talk to Kawhia locals during the next few days and try and build a picture of Langdon's intentions. New Zealand authorities have been helpful with daily updates, he says. He's especially interested in confirming the amount of stores the catamaran set sail with.

Wyler, for the moment, remains in her native Switzerland with an ill relative. Chapman says he's keeping her in the loop.

"Ariane is doing okay but this has been going on for two years now, trying to catch up with her daughter," Chapman says.

"It's cost her over $100,000 [mainly in legal costs] and she doesn't know if her kid is dead or not. None of us do."