Emergency services, who were alerted around 6pm yesterday, were amazed the man had survived, and with no major injuries.
Mr Potter called it a miraculous escape.
"How the hell he got out, we're still shaking our heads.
"That man should go and buy a Lotto ticket, but he hasn't got any luck left I don't think."
The man had been travelling from Cromwell to Queenstown when the accident happened.
"He came on to the Victoria Flats bridge and lost it. The trailer turned side-on, blocked the road near enough," Mr Potter said.
A St John Ambulance spokesman said one man was taken to hospital on status three condition.
Gibbston Highway was closed for about 35 minutes.