A fishing trawler collided with an overloaded cabin cruiser in Sydney's famous harbor on Thursday.

Four women and a man, all in their late teens or early 20s, died when the vessels collided in the dark. Nine others were injured.

All of the dead and injured were aboard the smaller boat, a 6-meter cruiser.

Sydney Ship Repair and Engineering, a boat repair company, also the owner of the cruiser, says it appeared to have been stolen for the trip.

Police are yet to release the identities of the dead.

Inspector Glen Finniss is with the Sydney Water Police.

"Two boats are moving on the water at night time. It's poor visibility at night time, obviously. There has been a collision involving two vessels and as a result of that people have lost their lives."

Australian Prime Minister Kevin Rudd has also commented on the accident, saying he is "stunned" by the tragedy.

Damage to the cruiser suggests it may have been rammed from behind by the trawler, a lobster boat heading out to sea.

The cause of the accident is still under investigation.

Deadly boat collisions in Sydney Harbor are rare. The most notorious one was in 1927, when a mail steamer collided with a ferry, sinking the ferry and killing 40 people.