Emergency services responded to reports that a ute had rolled over on the Moulamein Road in Moulamein, about 110 kilometers from Deniliquin, shortly after 12pm.
NSW Police said when they arrived they found a Toyota Hilux with extensive damage.
The six-year-old died at the scene while the four-year-old was treated by paramedics from both NSW and Victoria.
The surviving child was taken to Melbourne Children’s Hospital.
“The driver, a 31-year-old man, was taken to hospital for mandatory tests,” NSW Police said in a statement.
The police are investigating the circumstances of the accident