The Australian-registered Coral Adventurer became stranded after hitting a reef 100 kilometres east of the PNG city of Lae on December 27, 2025.
No one on board was injured, but investigators found the ship was travelling significantly faster than intended when it veered off course.
About 5.10am, the ship's chief mate attempted to adjust the planned course to Dregerhafen harbour on its digital navigation system, the Australian Transport Safety Bureau said on Thursday.
However, the vessel's autopilot would not complete the rerouting until a safety check had been completed, causing the ship to miss its first turning point and slow considerably.
Because of the lack of speed, the chief mate increased power to both engines causing the speed to rise to 8 knots, 2.5 knots higher than planned, ATSB chief commissioner Angus Mitchell said.
The ship returned to its planned course but did not turn as quickly as expected at the next waypoint, prompting the chief mate to turn off autopilot to correct course.
"At this point, in dark conditions and without navigational aids or lights ashore, the chief mate reported being unable to visually identify the surrounding topography," Mr Mitchell said.
After turning more sharply, the Adventurer continued past its planned track and grounded on the reef about 160 metres from the intended position, the preliminary report found.
The ship sustained hull damage and structural problems in the incident.
All passengers were disembarked three days later and the ship was navigated to a safe anchorage for investigations to begin.
Mr Mitchell said a final report was still being prepared and the investigation would assess CCTV footage and the ship's voyage data recorder.
"Investigators will also analyse human factors considerations, and will review the ship and its operator's passage planning and navigation procedures," he said.
"There will also be a review of the ship's emergency response procedures, and overall safety oversight."
The company offered passengers refunds for the affected leg of the journey or credits for future travel.
The grounding followed a previous mishap involving the Cairns-based vessel on October 25, when 80-year-old passenger Suzanne Rees died after allegedly being left behind on Lizard Island in far north Queensland.
