Emergency services were called to a commercial property in Gwehelog, Monmouthshire, at about 2.50pm on Tuesday, following reports that two men and a woman had been attacked by the animal.
A man, 57, from the Gwehelog area, was pronounced dead at the scene by paramedics from the Wales Ambulance Service. A second man, 19, believed to be his son, was airlifted to the University Hospital of Wales.
The teenager sustained critical injuries during the incident, a spokesperson for Gwent Police said.
A woman, 22, also from the Gwehelog area, sustained a serious injury to her leg, but her condition has not been described as critical. She was been taken to the Royal Gwent Hospital for treatment by the Wales Ambulance Service.
The spokesperson said the water buffalo was ‘destroyed’ following the attack.
It is not known exactly where the incident took place, but a farm in the area keeps water buffalo to make soap out of their milk.
Officers tending to the scene also received assistance from the National Police Air Service (NPAS).
