The search for missing yachtie Alan Langdon and his daughter, Que, has been completed by police.

Langdon, 49, and his 6-year-old daughter have not been seen since they left Waikato's Kawhia Harbour for the Bay of Islands on December 17.

Meanwhile, Australian child recovery expert Col Chapman has had a slow morning talking to locals in the seaside town as some continue to remain silent.

The police search has covered a wide area of the New Zealand coastline including the western coastline of the North Island, from Wellington to Cape Reinga and the eastern coastline from Cape Reinga to the Bay of Islands.

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The boat Langdon and his daughter are believed to be on is a 6.1-metre Wharram design catamaran, painted white, with white sails and the number T878 in black on the sail.

"There have been a number of unconfirmed sightings of similar vessels in the upper North Island area, but none of any significance since the new year," police said.

Police said it has been suggested that the boat was stocked for a voyage, but it is unknown what direction Langdon went after leaving Kawhia Harbour.

Langdon did not give anybody any notification of his intentions, police said.

Police have notified Interpol of the missing boat and its occupants, on the chance that the pair have sailed to Australia or the Pacific Islands.

The search has been managed by the Waikato Police Search and Rescue and the Auckland Police Maritime Unit with support from the Rescue Co-ordination Centre of New Zealand, Coastguard and the New Zealand Air Force.

Police said they would respond to any positive sightings or information that could come to hand.

Meanwhile, Chapman has spent the day in Kawhia canvassing locals, most of whom appeared to have closed ranks.

"I've talked to a dozen people but the town has closed around him a lot because he's a boy from the town and they've also only ever heard his version of events."

Chapman said there was surprise when he showed them information from his files.

The overall mood of the locals so far appeared to be of the view that Langdon had high-tailed it to Australia packing the catamaran's hulls full of supplies, enough for about 30 days.

Even the pair's bikes were on board, he said.

"They've noticed mood changes in him in the weeks leading up to it. They felt that he was a lot more secretive. Then some of them noticed that he was loading the vessel up considerably full of stores."

Chapman has also spoken to the builder of boats exactly like Langdon's who told him it was strong enough to withstand force 5 to 8 gale winds and capable of doing the transtasman trip 10 times in those conditions.

Search and rescue crews had told him that every part of Langdon's boat was also able to float, he said, meaning if it had suffered a mishap it would have been spotted along the New Zealand coastline.

"If that boat had a mishap on the way they would have seen the wreckage. It cannot sink. It's like a cork, it floats," Chapman said.

The only piece of evidence he was so far able to pick up was a photo of the conditions the day Langdon left, noting that the water was "like glass".

"I got told [initially] it was 20-25 knot winds, there was breaking waves, but talking to locals they said bad weather was coming. So he had a weather window."

He hoped to gather more information to pass on to police so they could make a formal request to Australian authorities to pick up the search.

Chapman is due to fly out of the country on Saturday.