The incident, which occurred in late January, is under investigation by the Transport Accident Investigation Commission (Taic).
"At the Port Chalmers freight marshalling yard about 1.25am on January 23, on the track leading into the south end of the marshalling yard, a remote-control operator and a rail operator were working to couple up a shunt locomotive to a rake of freight wagons (a set of coupled wagons).
"Both workers noticed shadows of something big moving towards them from the north," the commission said.
"They stepped out from where they were working in time to avoid what happened next.
"Another rake of nine freight wagons rolled uncontrolled from the marshalling yard, and the runway rake’s lead wagon ran into the locomotive. Reportedly there was moderate damage to the coupling area where the workers had been."
Rail and Maritime Transport Union general secretary Todd Valster said he was aware of the incident.
"It appears that the train or the rake of wagons wasn’t secured correctly, and that’s why it would have moved, and that would have been the people that were down there at the port at the time.
"So there are procedures, and sometimes if those procedures aren’t followed to the letter, then these things can happen."
There were no injuries.
Mr Valster acknowledged the incident could have been far worse.
"In the last 25 years, we’ve radically improved the practices in shunt yards to remove as much risk as possible.
"I dread the day when something bad happens, but I did live through an era when it was happening very, very frequently - that was the end of the 1990s."
The union worked closely with KiwRail to ensure lessons were learned from every incident, no matter the outcome, he said.
"It is good that there’s interest in it... that they are taken seriously and investigated, and people are taken through what we have as a just and fair culture, so they’re treated fairly,
providing they’re all up front about it.
"But it is a constant conversation. It’s a journey."
Processes strengthened - KiwiRail
In a statement late this afternoon, KiwiRail acting chief operations officer Paul Ashton said it was "a serious incident".
"Since the incident, KiwiRail has strengthened its verification processes to ensure safety-critical communication protocols are consistently followed.
"This includes enhanced local shunting procedures to improve operational safety and efficiency, and improved visibility tools and checks to help staff confirm systems are fully operational.
"KiwiRail has actively engaged with Taic throughout its investigation. As the investigation is ongoing, we have nothing further to add at this time."
