This is a strange accident resulting in nine injuries and five deaths. ABC News has a pretty detailed account of the accident; the photo above is from their site.
The accident occurred at night between a boat that was a mere 23' long and a fishing vessel named the Jordan. The smaller boat was also a commercial vessel. Judging by the name on its side it was a harbor service vessel.
One report I read stated the smaller vessel had been stolen. The ABC report didn't mention anything about a stolen vessel.
Fourteen people aboard a 23' boat (see picture) is a lot. But the boat -- believe it or not -- can handle it. As long as it's a short trip in calm waters, and nobody aboard is under the influence.
What I find mysterious is the condition of the vessel after the accident, as pictured in the above photo. If this is the actual vessel that was involved in the accident, it doesn't seem to be in very bad shape. It didn't sink. It didn't founder or capsize. And actually, from the photo, it looks like it withstood the collision quite well.
I've heard that what happens in a car crash is people get seriously injured by passengers who aren't wearing their seat belts. The force of the collision sends unrestrained bodies into each other, causing serious injury. Maybe this is partially what happened in Sydney Harbour.
We tend to think of injury and fatality aboard a boat as being a consequence of a foundering or sinking. In this case, it looks like the crash itself caused blunt force trauma to the passengers.