He had since received medical treatment.
Acting Manawatu area commander Detective Inspector Chris Bensemann said the officer had a cut around the top of his left eye and "numerous bite marks" across the top of his head.
He said he was appalled by the incident.
Photographs taken afterwards showed there "was blood everywhere", he said.
Mr Bensemann said police officers across the country were called to these incidents every day.
But what happened to the officer was "unexpected, unprovoked and without reason".
"He basically arrived and he was just confronted by this person who was standing beside his vehicle."
New Zealand Police Association president Greg O'Connor said he was concerned by the number of police officers who had been assaulted in recent weeks.
"What we find most frustrating is we will hear politicians and the police administration say the number of assaults on police is going down.
"The general assaults are going down, or the minor assaults, but serious assaults have gone up by 30 per cent since 2009."
Mr O'Connor said Tasers should be made available to and carried by officers at all times.
He said the association would be providing assistance to the officer who was bitten.
Mr Bensemann said the man involved was now in police custody and would face a number of charges.
His pitbull dog was seized by animal control officers following the incident.
Horowhenua District Council's customer and regulatory services manager, Mike Lepper, said that typically, once a dog was impounded, the matter was investigated.
The same would apply in this case, he said.
Animal attacks on cops
• Injuries to officers from January 1, 2011 to April 1, 2012:
• 62 injuries to staff arising from "being bitten or stung by animal, insect or spider"
• 34 injuries to staff arising from "being hit or struck by an animal, insect or spider"
