A nearby resident, who did not want to be named, said he heard this "almighty bang" and rushed out to look.
He said there was a car in the intersection which had been "spun around" and the motorhome embedded against the parked car.
Carterton fire chief Wayne Robinson said there were no major injuries.
The woman who had been knocked to the ground had been taken to hospital with "very minor" injuries.
He said after the collision at the intersection the motorhome had lost its steering and collided into the parked car.
"The wife got flung away from the vehicle - she's being checked out [at hospital]."
The owners of the motorhome, Rona and Bruce Bramwell of Masterton, said they were on their way to their daughter's 40th birthday in Whitby, Porirua.
Mr Bramwell reckoned he had sheered his front axle in the crash.
His motorhome was a "labour of love" that he had worked on for two years.
"We're all fine, the only people that were knocked around were the people in the car."
They still planned to make their daughter's birthday.