A woman was held-up at knifepoint for more than an hour at a South Auckland mall before the alleged offender dropped his weapon and was arrested by police.

Westfield Manukau was evacuated after a man grabbed a stranger and held a knife to her throat, sparking lengthy police negotiations.

The 34-year-old victim, who is from Christchurch, had been at the shopping mall with her daughter.

Counties Manukau Area Commander Inspector Adam Pyne said the victim was not known to the offender and was "picked randomly".

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Police were called at 8.50am to the hostage situation and were at the scene within minutes, he said.

"The first officer on the scene immediately began negotiations."

Police negotiated for about 40 minutes before the man dropped the knife, which had a 12.5cm blade, and released the victim.

The knife had been held to her throat and the armed offender squad had been called, he said.

"It took a long time but eventually they were able to diffuse the situation," he said.

"The victim in this matter is understandably shaken but fortunately was not harmed by the alleged offender."

It was obviously an extremely distressing situation, he said.

"I want to acknowledge the bravery of the victim in this incident, who remained calm in a stressful and traumatic situation and we are providing her with all the support we can."

The alleged offender has been arrested and will appear in the Manukau District Court tomorrow charged with kidnapping.

Pyne said the incident was unusual.

"I've not known this situation before ... it's not a common thing at all."

Mall security spoke to him prior to the mall opening, he said.

"When he grabbed the victim security helped evacuate the food court and then the wider mall".

Pyne commended his staff for diffusing the "volatile" situation.

He said inquiries were ongoing as to the alleged offenders motivation.

Part of the police investigation would be whether the man was under the influence of drugs or alcohol but Pyne said it was too early to comment on that.

Pyne said the alleged offender was not making any other overt threats to other people in the area.

A bullet was found outside the mall, but Pyne said that was not related to the incident.

Pyne could not say if the man was known to police as the matter was before the courts.