Alan Langdon's transtasman trip would have been slow going, yachting expert says

Police are now searching shorelines north of Auckland for a yachtie and his six-year-old daughter who have been missing for 11 days.

A yachting expert says a fleeing yachtie's sailing trip with his young daughter would have been slow going with big weather risks as he crossed the Tasman.

Alan Langdon would have averaged about 50 nautical miles a day after leaving Kawhia Harbour on his way to the New South Wales coast, said yachting expert Alan Ward.

The Waikato yacht club member and former commodore said even for a boat of Langdon's size that was slow going.

Langdon, 49, and his six-year-old daughter Que were believed to have made the journey on his 6.1m Wharram-design catamaran.

Alan Langdon and his daughter Que. Photo / Supplied

The pair left Kawhia Harbour in the Waikato on December 17 and the pair were not seen or heard from until today when a local in Ulladulla, NSW, reported seeing them to Australian authorities.

Ward estimated the journey to Ulladulla would have been about 1300 nautical miles meaning Langdon would have averaged about 50 nautical miles a day in the 26 days he was gone.

"Fifty miles over 24 hours on average is very slow.

"Even in that boat it's very slow, but it all comes back to conditions."

Most 30-35ft (9-10m) sailing boats could do about 120 nautical miles in a day, but it was not clear whether Langdon may have had to stop for stretches due to rough seas.

He said the Tasman could be incredibly rough and if conditions turned it would have been hard to navigate the seas in such a small catamaran.

"It can be a nasty bit of water," he said.

"I did a transtasman race in a an 11m boat and, quite frankly, I wouldn't want to do it in anything smaller."

The pair were missing for 26 days, but believed to have arrived in Ulladulla two or three days ago, said child recovery expert Col Chapman.

Chapman had earlier told the Herald locals had seen Langdon packing his catamaran with enough supplies to last about a month.

Even the pair's bikes were put on board.

"It's easy to stock up, and particularly if there's only two of them and she's a little kid - food wouldn't be a problem," Ward said.

"Even though [the boat] is small, a few canned things, a bit of rice and spaghetti you'd certainly survive alright, it wouldn't be a problem."

He said the bigger problem would have been having enough fresh water on board to last the distance.