Police are warning against posting serious allegations online after a photo of "scum" attacker spreads on Facebook.

A bid to identify a supposed attacker after a drunken fight has sparked a backlash online, with one who shared the post admitting "trial by social media is cruel".

Several Facebook posts, which included a photo of the woman with a man in the background, called the man "scum" and claimed the woman suffered a broken back after he allegedly attacked her in central Christchurch.

The posts sparked a flurry of debate and counter allegations. Police warned against posting serious accusations online, saying they already know who he is.

One person who shared the post later removed the photo admitting "trial by social media is cruel and photos and stories may never go away".

"We all must try to remember how to balance our use of social media with situations like this, as well as our own liability," they said.

Inspector Derek Erasmus said police attended the fight, which took place in several central city streets. It involved a group of six people either celebrating or commiserating the Rugby World Cup final result shortly after 11am on Sunday.

"All were intoxicated," he said.

Police were not aware of any serious injuries as claimed in the post.

"Those involved appeared intoxicated and three people were arrested. An officer was struck in the face during the arrest, but was able to continue their duties after being assessed," he said.

Police gave pre-charge warnings for disorder to a 33-year-old man, 31-year-old man and 39-year-old woman.

The 39-year-old woman was charged with assaulting police and was due to appear in the Christchurch District Court on Friday.

She contacted Stuff after publicity about the posts. She said she first put the picture on Facebook asking only who the man was. It was friends who re-posted it who made the allegations of assault. She had since asked them to remove them.

She saw the man in the police cells on Sunday, but wanted to find out his identity for herself so she could lay an assault complaint, she said.

"I am seriously injured," she said.

The woman planned to defend the assault on police charge. A verbal exchange of insults with the man's girlfriend sparked the incident. She accidentally hit the police officer while clashing with the girlfriend, she said.

Erasmus said earlier that concerned members of the public and the media contacted police about the online posts.

"The two people in the picture, we know who they are and have dealt with conflicting arguments about what happened," Erasmus said.

He urged caution about posting allegations on social media.

"There may be legal consequences including under the Harmful Digital Communications Act 2015," Erasmus said.

Stuff has not published the photo for legal reasons.