Police were called to the crash on Patunamu Road, near Tuai, 52km north-west of Wairoa just after 11am on Monday.
They said the driver appears to have over-corrected on a moderate bend, before losing control and rolling at least twice.
A 30-year-old man is in a critical condition with head and chest injuries, while a 26-year old man is in a serious condition with chest injuries.
The other passengers escaped with minor injuries.