WorkSafe's investigations manager for South Island, Steve Kelly, said an investigation found there were two pieces of wire rope acting as a barrier around the trailer's top deck.
"Testing showed these ropes were severely corroded. Our investigation also found that limited space on the trailers meant that workers had to turn sideways and push up against the wire ropes when entering and exiting vehicles and strapping down the wheels.
Kelly said the business' incident register had recorded four falls from heights on trailers between 2017 and 2018.
Previous reports prepared for the business also highlighted the need for proper maintenance of the wire ropes.
"The risk was very much on the business's radar. They should have ensured that these wire ropes were inspected regularly and periodically replaced by a competent person to ensure they remained fit for purpose."
Car Haulaways Limited was ordered to pay a fine of $279,000 and reparation of $90,000 to the victim's family.
Edge protection must be inspected, maintained and routinely replaced to ensure it can effectively keep workers safe from a fall from heights, Kelly said.