A female police officer was taken to Middlemore Hospital after she was assaulted near the Ōtāhuhu fan zone.

A female police officer was knocked unconscious in the Auckland suburb of Ōtāhuhu following Tonga's Rugby League World Cup win on Saturday night.

A witness said the policewoman was attacked around 10pm near the fan zone.

She appeared to be in a lot of pain and was holding her arm, the witness said.

DAVID WHITE/STUFF Police detain a woman during fan celebrations in Ōtāhuhu. The woman was later released to the care of her friends

Inspector Rod Honan said she was taken to hospital with a head and arm injury "after being knocked unconscious by an unknown man from behind".

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"Someone knows exactly who committed this cowardly act and we urge the offender, or anyone with information, to come forward.

"We know a number of people would have seen the offender.

"Police staff work tirelessly every day to keep our community safe and violent acts like this against our officers are unacceptable.

"The officer has been discharged from hospital today and will continue to receive support while she recovers at home," Honan said.

Ōtāhuhu town centre was heaving with fans after Mate Ma'a Tonga snuck a 24-22 victory against Lebanon in Christchurch.

There had been a large police presence in the area after riots resulted in the arrests of 53 people last weekend.

One person had been arrested in south Auckland during Saturday night's celebrations as of 10pm.

In Christchurch there were no incidents of concern and no arrests.

One person was evicted from AMI Stadium for intoxication, but the level of alcohol consumption was low, Canterbury District Metro Area Commander Superintendent Lane Todd said.

The assault follows a security guard being king-hit outside the Tonga vs Samoa game in Hamilton earlier this month.

In the other quarter final, New Zealand experienced its worst ever defeat, losing to Fiji 4-2 in Wellington.