The Port Authority of NSW said in a statement it responded on Saturday to "a fire onboard a recreational vessel moored at Double Bay". Fire and Rescue NSW received multiple Triple Zero calls shortly before midday and dispatched crews who tackled the flames from several directions.
The fire tore through the cruiser, gutting it completely. A Port Authority vessel with firefighting capabilities was deployed to assist.
"There were no passengers onboard and no reported injuries," the Port Authority said.
The Port Authority worked with NSW Police and Fire and Rescue NSW to manage the incident. An investigation into the cause of the fire is expected to follow.