The incident happened as the 297,000-dwt German-owned bulker Julia N (built 2012) was being positioned in June.
During the retrieval of a tow line, a seafarer's right leg became entangled and he was dragged four metres across the deck, losing his foot.
The Australian Transport Safety Bureau (ATSB) said investigators could not interview the injured seaman.
"There is no clear evidence to determine the actions of the second mate and how they were interpreted by the tug's crew as a positive signal to retrieve the tow line," the report said.
The
