The accident took place at about 3.30pm on Friday on State Highway 2 near Raupunga. The grandmother of five was the only person in the car.
Police were investigating the accident.
"She'd just turned 65 and she was really excited about that," her sister said, adding that she did not plan to retire yet.
Ms Shadbolt had one son, who was now living in Australia.
"She was a kind, generous and giving person," Lyn said.
Another sister, Robyn, died a week earlier following an illness.
Ms Shadbolt's funeral would be held in Wairoa on Friday, followed by a private cremation.
The provisional road toll for the official Labour Weekend holiday period was five - two more than the same period last year.