The ship of 520 tons was built on the River Thames, England in 1781. She was first called the "Berwick.". She had been built to work in the Baltic Sea to carry timber. The Royal Navy bought it to use as a store ship. She was refitted for the trip to Australia, and renamed "HMS Sirius". Lieutenant Philip Gidley King wrote that the ship was named after the star in the Orion constellation.She was armed with six carronades, short cannons used to fire large objects to smash ships. She also had four six-pounder guns. There were another ten six-pounder guns in the cargo hold to be used to protect the new settlement. HMS Sirius was about long and wide. Her hold was about deep.