New Zealand rescue authorities on Friday conceded hopes were fading for six Americans and a Briton whose vintage wooden yacht went missing in rough seas more than three weeks ago.

Searchers looking for the schooner Nina had shifted focus from the open waters of the Tasman Sea to scouring the rugged shoreline of New Zealand's upper North Island for survivors, life rafts or debris, the Rescue Coordination Centre New Zealand said.

"We're looking at the possibility that if they've abandoned to a life raft at some point, where that life raft may be. Is there anywhere on the New Zealand coast where it may have fetched up?" rescue coordinator Dave Wilson told Radio New Zealand.

The 21-metre (70-foot) Nina, built in 1928, set off from the North Island bound for Newcastle, Australia, on May 29 but has not been heard from since June 4, when a major storm swept the area.

Friends and relatives raised the alarm on June 14 and all efforts to contact the vessel have since failed, even though it has two satellite communication systems, an independent emergency backup and two VHF radios.

An extensive search by a New Zealand air force place also found no sign of the yacht in the Tasman Sea.

"As time progresses and there's no sightings and no communications then obviously we hold grave fears... for there to have been no communications from any of those systems is what's leading us to be quite concerned," Wilson said.

The missing crew comprises six Americans -- three men aged 17, 28 and 58, and three women aged 18, 60 and 73 -- along with a 35-year-old British man.

None have been officially named, although media have reported they include owner David Dyche III, his wife Rosemary, son David and 73-year-old Evi Nemeth.

The family reportedly left Florida in 2008 to circumnavigate the world, travelling through the Caribbean, Central America and French Polynesia before arriving in New Zealand.

Wilson said the yacht's skipper was a professional mariner with extensive experience.

He said the Nina, a well-known vessel in yachting circles, was old but "very sturdy".

"It's made of solid timber, built in 1928 and ... done a lot of oceanic sailing previously," he said.

In 1928, the Nina became the first US vessel to win the famous British Isles Fastnet race, according to an entry on the website sailblogs.com by Rosemary Dyche.

She describes the schooner as the flagship of the New York Yacht Club after World War II before her husband bought it in 1988, detailing its restoration as "a labour of love".

Wilson said if disaster had struck in early June it was possible there was no longer any wreckage floating in the notoriously rough Tasman Sea.

"It's an extensive area (and) we're talking about three weeks of drift," he said.

"So if there's something catastrophic that has happened, given the sea conditions and the wind, it's likely any debris on the surface would have dissipated fairly quickly."