The Rena's crew would have been "terrified" when they issued a mayday, forcing their evacuation from the stricken ship, the maritime workers union said.
The 17 mostly Filipino crew members issued their call when the ship crushed the reef on which it was resting in the early hours of yesterday and shifted back a few degrees from its 10-degree list.
Maritime Union general secretary Joe Fleetwood said he did not accept official advice that the mayday was precautionary, and said the crew would have been fearing for their lives.
"It wouldn't have been a precautionary thing. As soon as you hit that button you're in distress. They probably feared for their safety, thinking this vessel's going to snap in half or roll over and kill everyone on board.
"I guarantee they would have been terrified."