What would you do if you witnessed an attack like this?
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"Bystander apathy'' is being blamed for the failure of anyone to step in while a pregnant woman was bashed in front of a crowd in central Rotorua.

The woman, whose pregnancy was not showing, was dragged from a dairy and attacked - including being kicked and stomped on - in front of about 20 people in Pukuatua St about 10.30am on Wednesday.

At least two witnesses called police but none stepped in to stop the assault, something clinical psychologist Barry Kirker put down to "bystander apathy'' - a phenomena which has been studied since the 1950s.

Those studies showed people who were part of a large group were less likely to intervene in such an incident if no one else did, something known as diffusion of responsibility.

"Everyone thinks 'well why should I do it, there's 20 other people here', so then nobody does,'' Mr Kirker told APNZ.

"There's no pressure for anyone to take any accountability to do it.''

Once someone did step in, others tended to follow, and it was more likely people would step up if only small numbers were involved.

"It just needs someone to take the lead,'' he said.

"Someone needs to break free of that social phenomena of the bystander apathy and stick out, be courageous.''

Fights had the added uncertainty of the attacker perhaps being armed, or having supporters nearby, Mr Kirker said.

Parents tended to encourage their children not to get involved due to safety fears, and there had also been cases of good Samaritans being killed when they intervened; Austin Hemmings was stabbed to death when he tried to save a woman being attacked in central Auckland in 2008, while in 2009 a Melbourne man was repeatedly stabbed when when he tried to break up a fight outside a nightclub. Two Thai men had been charged with his murder.

Mr Kirker was in no doubt as to what he would do if he saw such an incident, although his stance had changed slightly since becoming a father.

"I certainly wouldn't hesitate - that's because I'm aware of those phenomena,'' he said.

"I'd try and take a leadership role and say `you, you, come on, let's go, if we work together we can do something whereas one of us might just get hurt'.''

The Rotorua incident flared when the victim was followed into a dairy by a young woman screaming and yelling abuse at her, including calling her a "black b****''.

Witnesses said the attacker was accusing the victim of "sleeping with her man''.

When asked to leave by the dairy owner, the woman went outside and continued the abuse from the doorway. The dairy owner, who did not want to be named, said the victim was terrified and did not want to leave the shop.

A witness at the bus stop said: "We saw the chicky nutting off at the girl in the shop telling her to come out. The girl wouldn't come out so she went into the shop and started slapping the girl around and dragged her out by the collar of her jersey and pulled her across the ground.

"She dragged her around the corner and started assaulting her.''

The victim, who was treated at Rotorua hospital for facial injuries, said she had never seen her attacker before and did not know why she was set upon.

Andre Jansen manages the shop next to the dairy and heard the abuse start. The victim was going about her business alone and did not appear to provoke the other woman, he said.

Acting area commander Inspector Ed van den Broek said police had good photos of the alleged offender were following positive lines of enquiry. However, they still wanted to hear from anyone who saw the incident or who had information on it.

He urged people to call police if they saw such an incident, and think twice before intervening.

"If you are feeling confident and make a judgement that you can step in, then do so, but people have to be cautious as you don't know what you are dealing with,'' he said.

"People could be armed with weapons or there could be supporters of the offender in the crowd that may take offence.''

Once police were called witnesses should note details that could help in any investigation including descriptions of those involved and vehicle registration numbers.

Filming or taking photos of the violent situation should be done discreetly and only if there was no personal risk, he said.

Rotorua police had increased beat patrols in the central city over the holiday period and that would continue until the end of the school holidays.