Traffic constable Steve Forster said the high winds that had hit the region yesterday had caused the trailer to roll.
"It's bowled it in the wind," he said.
He warned motorists not to venture out in such conditions.
"If you don't need to go out then stay at home where it's safe."
Shortly before the truck incident, police issued a warning about wind gusts urging those with high-sided vehicles, riding motorcycles or in campervans to take extreme care near Mt Bruce as it was notorious when winds reached high speeds.
Mr Forster said a tree fell on to a car on State Highway 2 but no one was injured.
Down the road closer to Masterton, traffic policing sergeant Chris Megaw was stopping and warning campervan drivers to the danger ahead. "I've just been telling people to take it easy and not drive at speed," he said.
It was not easy to close the main highway between Eketahuna and Masterton, Mr Megaw said.
Meanwhile, a fire crew was called to Upper Waingawa Rd, Kaituna, just before 10am when two trees came down, bringing down power lines.
Station officer Tony Gibbs said one tree came down on to high-voltage lines that set the tree alight and brought down the live wires. He said the fire crew had to wait for power staff to turn off the power.
Meanwhile, by 2.30pm yesterday, 2300 Wairarapa customers were without power including residents east of Masterton to the coast and just north and south of the town, as the winds battered the region.
Powerco Network operations manager Phil Marsh said extra crews had been called in to restore supply and make lines safe. Around 50 field staff were working in affected areas.
He urged people to keep clear of damaged lines and report them by calling freephone 0800 272727. " Keep yourselves, your children, pets and livestock well away from and call Powerco."
