Two men had to have surgery after an attack by a dog in Wanganui on Friday night - an attack so vicious that police could end it only by shooting the dog.
Acting Senior Sergeant Andrew McDonald said the men suffered severe bites in the attack.
Emergency services could do nothing until police shot the dog, which allowed them to get in and help the men.
The dog owner and another man were arguing in the driveway of a Bamber St house at around 7.30pm on Friday.
Mr McDonald said police were unsure why the man's dog - a large crossbreed - attacked them.
The owner was fighting with his dog for about 10 minutes before help arrived, Mr McDonald said.
"The owner couldn't gain control of the dog and asked police to destroy the animal. We had no option but to put the dog down immediately."
The dog owner suffered deep lacerations to both arms and legs. Both men have had surgery and are still in Wanganui Hospital. They are in a stable condition, a spokeswoman said.
Whanganui Police are speaking to witnesses, and are keen to know what caused the dog to attack its owner and the other man.