Due to his injuries, police had been unable to interview the man and were eager to speak to anyone who may have seen the attack. "He might have been on the ground for a short time before someone came to help him so people may have driven past and seen the situation."
At this stage it was unclear what caused the dog to attack, Mr Megaw said.
The dog's owner was co-operating with police and Masterton District Council.
It was too soon to indicate what the repercussions for the owner might be, but it was possible charges could be laid, Mr Megaw said.
"At the moment we are just trying to work out what has taken place."
Council spokesman Sam Rossiter-Stead said the dog was registered and had not been involved in any previous attacks.
The dog would now be classified as "menacing by deed", which meant it would have to wear a muzzle in future, and it would be taken out of the area for assessment.
The fate of the dog would not be determined until police and council investigations had been completed, Mr Rossiter-Stead said.
Anyone who has information about the attack should contact Masterton police on 06 3700300.