A woman, an 11-month-old boy and a two-year-old girl are in hospital after they were struck by an SUV with a trailer.
Police were called to the crash on Otamarakau Valley Road in Pukehina in the Bay of Plenty about 2.30pm. Police believe fatigue may have been a factor in the crash.
They said the two children, who were in a stroller at the time of the crash, were flown to Waikato Hospital in a critical condition.
The 11-month-old boy has since been flown to Auckland's Starship Hospital.
The 33-year-old woman who was pushing the stroller was also taken to Tauranga Hospital, where she was in a serious but stable condition.
The two-year-old girl, who was earlier in a critical condition, is now in a serious but stable condition in Waikato Hospital.
The driver of the SUV was unharmed, and was assisting police.
The Serious Crash Unit was investigating.
Western Bay of Plenty Mayor Ross Paterson told Checkpoint with John Campbell there were some areas off the rural road where a stroller could go, but in other areas people had to walk on the road.
"It has been a tragic accident to involve such a young family and our thoughts go out to the family at this time, and it's just terrible for them to suffer this sort of thing."