WELLINGTON (AFP) - New Zealand search officials are to re-evaluate their search for six Americans and a Briton missing at sea for nearly four weeks after another fruitless aerial sweep on Sunday.

The seven were last heard from on June 4 when they reported they were sailing into a storm while en route from New Zealand to Australia.

The New Zealand Rescue Coordination Centre believes their schooner Nina sank and their search is focused on looking for a liferaft or debris off the West Coast of the North Island.

A New Zealand Air Force Orion spent nearly eight hours on Sunday conducting a "lower, slower search than previous searches," but reported no trace of the 21-metre Nina nor its crew, centre spokeswoman Sarah Brazil said.

Since the alarm was raised on June 14, 10 days after the last communication with the yacht, land and sea searches have covered more than 500,000 square nautical miles, without success.

Ms Brazil, the spokeswoman, said search officials would re-assess the situation overnight and decide their next move on Monday morning.

The 85-year-old wooden vessel Nina, owned by American David Dyche, 58, was heading for Newcastle, Australia.

Also on board were his 60-year-old wife, Rosemary and their son David, 17, along with a 28-year-old American man and a 35-year-old Briton, an 18-year-old American woman and noted American computer systems authority Evi Nemeth, 73.

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".