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Constable Andy Denny, of Balclutha, said emergency services were called to a crash on the Kaitangata Highway shortly after 9.30am.
He said it appeared the driver misjudged the height of the bridge, and the force of the top of the truck hitting it caused the truck to spin around before hitting the roadside guardrail.
The road was closed for about 45 minutes while the Balclutha Volunteer Fire Brigade worked to untangle the truck from the roadside guard rail. The crash caused minor damage to the rail.
Const Denny said police inquiries into the crash were continuing, but fatigue may have been a factor.
Police were seeking information about a similar incident involving the same railway bridge shortly before the crash, he said.
"Police were advised that a ... truck carrying a load of timber also collided with the bridge earlier that morning," Const Denny said.
"Timber caught the top of the bridge and caused some wood to fall off the truck."
The incident was not reported and Const Denny asked for witnesses or the driver concerned to contact the police. The exact time is unknown.