"Although the knife was present, we don't think it had any part in the assault," Mr Sutherland said.
The women had not been patrons of the restaurant but staff had intervened.
A witness said she had been alerted to the incident when she had heard lots of "screaming and carrying on".
According to the witness, one of the women had left the scene and returned with what appeared to be a boning knife. She then left after the fight. An ambulance was called to the scene.
A St Johns spokesman said the woman was treated at the scene for minor injuries.
While the victim was being treated in the ambulance the offender returned to McDonald's for the second time, and began screaming abuse at the victim, and at police.
She was then arrested.
McDonald's would not comment as it was a police matter.