"Unfortunately, other people in close proximity of the scene were affected by the spray but no one suffered serious effects from it," police said.
Three men are now facing charges following the incident.
Two of them were arrested for assaulting police and resisting arrest, and now face charges for those offences.
One of them was also charged with trespass.
A third man was arrested for obstructing police and was given a formal caution.
The witness said police officers as well as the public got caught in the spray. She could see officers having their eyes doused in water, too.
But by the time she arrived she only saw the aftermath of what had happened.
The witness, who was a visitor to Hawke's Bay, said a woman who was helping to hand out jugs of water was telling people to move along and that there was "nothing to see here".
The witness said she wondered if a member of the public had got the pepper spray off police and used it, or whether an officer had used it not realising it would affect other people too.
Pepper spray is formally known as oleoresin capsicum spray. The spray is listed as a restricted weapon under the Arms Act 1983.
The spray irritates eyes, causing tears and pain.
The inflammatory effects cause people's eyes to close, taking away their vision.
This allows police to break up crowds, restrain people and make arrests.