Pitman Racing director Michael Pitman, of Christchurch, said he was driving home on State Highway 1 from the Wingatui Boxing Day Races on Monday.
About 6.10pm, between Waitati and Waikouaiti, he pulled over to let a following vehicle pass.
"I hit a soft patch and it just dragged the back of the truck off the road ... Basically, it was like quicksand," Mr Pitman said.
The truck rolled on to its passenger side, blocking the highway.
"It all happened in slow motion," he said.
He was able to walk away from the crash.
Mr Pitman said he felt incredibly lucky on several counts — that both he and his passenger had been strapped in, and that the vehicle had not rolled any further.
They were also fortunate to receive help from many people at the scene.
He also praised the work of police and firefighters, whom he described as "absolutely outstanding".
Firefighters removed the truck’s back door to free the horses, which then walked on to the motorway after being trapped for about two and a-half hours.
"It was stressful for all of us," Mr Pitman said.
The horses were transported to stables in Waikouaiti by horse floats and Mr Pitman was confident they would be fine once they had a bit of time to recover.
It did not appear there was a lot of damage to the truck, which has been removed by a towing company, although it would be some time until the damage could be fully appraised.
"I’m just very, very lucky," he said.
Mr Pitman and his passenger were treated for minor injuries at the scene by St John and were not transported to hospital, as initially reported.
A St John spokeswoman said two ambulances, a rapid response unit and a manager attended the scene. Fire crews from Waikouaiti and Willowbank stations also attended.
The highway was reopened about midnight.
