Yesterday, Constable Jason Braid said the horse truck ramp ramp was hinged to the truck and served as a door in an upright position.
The ramp hydraulics appeared to have "malfunctioned" when Mr Anderton was preparing to lower the ramp to load horses to transport to Wingatui Racecourse for training.
The truck was owned by Mr Anderton and no horses were injured, Mr Braid said.
Mr Anderton was taken to Dunedin Public Hospital by ambulance in a critical condition, he said.
WorkSafe NZ was investigating.
Otago Racing Club chief executive Andre Klein yesterday said Mr Anderton was among the "backbone" of the Wingatui racing community and one of the most respected young trainers in New Zealand.
"Steve is regarded as one of the most professional trainers and a great bloke as well, a really, really good guy."
It was a "dark day" for the close-knit racing community, he said.