The two women were friends and worked together as cleaners, he said.
The incident occurred in Salisbury Rd, in Birkdale.
The incident began just after 4pm when a car the women were in came to an abrupt halt in the middle of a roundabout.
Witnesses said one woman jumped from the vehicle and began chasing the other, at one stage throwing a rock at her. Punches were thrown, and one woman was screaming at the other, "you shouldn't have done it", they said.
People nearby had tried to calm the situation, they said.
A 17-year-old resident who ran to the scene after seeing the drama unfold said the injured woman was in a great deal of pain.
A man who was leaving a nearby dairy said he heard a "heated argument" with quite a bit of swearing.
Another person arrived at the scene to find the injured woman propped herself up against a fence, he said.
"The first thing I did was call out to one of the customers at the dairy and told them to call the police."
A St John Ambulance spokeswoman said the injured woman was taken to Auckland Hospital with moderate injuries.