A man has been thrown from his boat after it slammed into a bridge in a busy waterway in Mandurah.

The 4m vessel was “launched into the air” after colliding with a fishing jetty beneath the Mandurah Estuary Bridge about 7pm on Thursday.

One of the two people aboard was understood to have been knocked unconscious, slipping in and out of consciousness before emergency crews arrived.

Marine crews, St John Ambulance and the RAC Rescue helicopter responded to the crash, which happened near the Dudley Park boat ramp.

Paramedics treated the man at the scene before he was taken for further treatment at Peel Health Campus.

A witness described seeing concerning behaviour from the injured man.

“He looked concussed as he originally passed out after the incident,” they told the Baldivis Times.

“Then he got up and said he was OK but he stopped replying and that’s all I know. The boat just started drifting after.”

Police are asking witnesses who saw the incident, or the vessel prior to the collision, to report it to Crime Stoppers on 1800 333 000.