A fishing trawler collided with an overloaded cabin cruiser that apparently was on a nighttime joyride in Sydney's famous harbor Thursday, killing five people and injuring nine, authorities said.

Four women and a man — all in their late teens or early 20s — died when the vessels collided shortly before 3 a.m., police and hospital officials said.

The U.S. consulate in Sydney said one of the victims was a U.S. national, but spokeswoman Alison Barnard said no further details would be released — including whether the American was among the injured or dead — until the person's family had been notified.

All of the dead and injured were between 18 and 31 and were aboard the smaller boat, a 7-meter (23-foot) cruiser owned by a boat repair company that said it appeared to have been stolen for a trip.

Police declined to speculate about the cause of the collision, but said alcohol, overcrowding, speed and whether the victims were wearing lifejackets would be among elements investigated.

Deadly boat collisions in the harbor of Australia's largest city are rare, and the accident drew comment from the prime minister and other top leaders.

"This is every parents' nightmare," Prime Minister Kevin Rudd told Fairfax Radio. "I just am stunned by this."

Police said they would investigate why the cabin cruiser — licensed to carry eight people — was overcrowded and being used for social purposes at such an hour.

The crash occurred near Bradley's Head in a favored sightseeing area of the harbor, home to Sydney Opera House and Sydney Harbor Bridge landmarks. Once a bustling commercial port, the harbor is now dominated by pleasure craft, passenger ferries and cruise ships.

John McPherson, general manager of Sydney Ship Repair and Engineering, which owns the cabin cruiser, said it had been moored in suburban Balmain and that it was not on company business at the time of the crash.

"It appears that somebody has decided that they are going to take our boat for a joyride and this horrible event has happened," McPherson told Australian Broadcasting Corp. radio.

Police said the fishing vessel, a lobster boat, was heading toward the harbor's passage into the open sea and the cruiser was apparently returning toward the inner harbor.

Senior Inspector Glenn Finniss said the cruiser appeared to have been struck near the center of one side, but there was surprisingly little damage given the death and injury toll.

According to Dr. Ray Raper, an intensive care specialist at Royal North Shore Hospital, one of the hospitalized survivors had a life-threatening brain injury.

The most recent fatal crash in Sydney Harbor was in March last year, when a passenger ferry plowed into a pleasure boat under the harbor bridge, killing three people. The most notorious was in 1927, when a mail steamer collided with a ferry, sinking the ferry and killing 40 people.