The man broke both legs, fractured three vertebrae, 10 ribs, his sternum, two tendons in his hand and lacerated his lung.
He was hospitalised for three months, and is unlikely to work again as a stevedore.
No safety rail was in place where he was working. POAL had not told stevedores to stop using unlocking poles to remove twist-locks, despite identifying that they were hazardous.
Maritime NZ director Keith Manch said the sentence reflected the seriousness of the incident.
"There were multiple failings of procedures and communication in this case and the long-term effects for the injured man have been devastating," Mr Manch said.
"All workers have the right to safe workplaces and to go home healthy at the end of the day."