"They have crossed the centre line, hit a ditch on the other side of the road and the vehicle has rolled. Alcohol and speed were not factors," he said.
The crash happened as the Masterton family, on their way to stay with family or friends in Taumarunui, was travelling north about 5km south of Owhanga.
The girl's death has devastated the staff of the Kohanga Reo, said office staffer Kathleen Albrett-Rimene who said Kahui was "the life of the Kohanga".
"She was a bright little girl who was top of the class, always asking questions."
Kahui had blended in well with the other children and was a strong student, especially in waiata and within the Maori language. Staff were confident she would have moved onto Kura and become a leader there, Ms Albrett-Rimene said.
The wider whanau and friends are heading north to be with the family and although it is early days yet it is likely an acknowledgement of Kahui's time and achievements at the Kohanga will be held once school resumes.
Kahui has an older brother Jayde.
Tributes to Kahui have been posted on Facebook.
She was one of two children killed in separate crashes in the country last weekend, the other being an 11-month-old girl who died following a crash near Raumati South on Saturday.