The 1928 wooden schooner, the Nina, left a northern port in New Zealand at the end of May, but stopped issuing communications about a week later, a maritime safety authority said.
The sailboat was headed to Newcastle, Australia, though when it was scheduled to arrive was unclear.
It dropped out of communication about 400 miles northwest of New Zealand in the Tasman Sea, CNN reported.
The boat is equipped with a tracking device, a satellite phone and an emergency buoy, and none of the technology has been used since early June, CNN said.
A reconnaissance plane conducted two extensive sweeps over the sea, but did not find the vessel.