About 50 people were treated by emergency medical workers on Monday after a Latam Airlines flight bound for Auckland, New Zealand’s largest city, experienced what the airline called “strong movement” caused by a “technical problem.”

Twelve people, one of whom was in serious condition, were taken to three hospitals, Auckland’s ambulance service said. Latam, a Chilean airline, provided no specifics about the technical problem that it said had caused the disturbance.

The plane made a “violent drop” for just a second or two, said Brian Jokat, who was on board Latam Airlines Flight 800. Aircraft tracking information from Flight Aware showed a gap of roughly an hour for which no data was available.

“The plane just dropped,” Mr. Jokat, 61, said. “It felt like coming over the top of a roller coaster and heading down — and then it straightened up immediately.”

A jolt roused Mr. Jokat, who was wearing his seatbelt, from a nap, and in the next instant the plane was in what felt like free-fall. He saw the passenger next to him being thrown up to the ceiling of the plane, and then come crashing back down.

“Anyone who wasn’t in a seatbelt was thrown,” he said. “You could not have not been thrown.”

He added that there were no prior announcements that passengers should fasten their seatbelts, as would normally precede turbulence on a flight.

The plane, a Boeing 787-9 Dreamliner, was met at Auckland International Airport by 14 emergency vehicles, including seven ambulances, according to the city’s ambulance service, Hato Hone St. John. Ambulance crews treated about 50 people at the scene, including the person in serious condition; the others were in “moderate to minor condition,” the service said.

Six people had been taken to the emergency department at Middlemore Hospital in Auckland and four others were on their way, according to Rob Harley, a spokesman for the hospital. He said the six patients at the hospital had injuries including cuts, abrasions and lacerations, but that all were able to walk — “in other words, not life-threatening.”

The flight had been scheduled to continue from Auckland to Santiago, the Chilean capital, later Monday. Latam said that leg of the flight had been canceled and that passengers bound for Santiago would be flown there on Tuesday.