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Acting Senior Sergeant Chris MacAulay said it appeared the truck was ''unmanned'' at the time and had rolled across the four-lane highway, before hitting the wall.
A serious crash unit team was investigating the incident.
No-one was injured, which was lucky, he said.
Southbound traffic was reduced to a single lane for more than an hour, causing delays for motorists.
Nikki Scott, who lived nearby, said it was ''amazing'' no-one was hurt in the incident on such a busy road.
Her nephew, Jono Scott, had driven past the crash site just minutes after the incident, she said.