Update: Rotorua school lockdown over (+video)

Tensions were running high as worried parents gathered outside a locked-down Western Heights High School.

A telephone threat was received by the school about 12.30pm today, forcing the evacuation of students and staff to the perimeter of the playing field, away from buildings.

Calls "of a concerning nature" were made to 27 schools around the country yesterday following six threats yesterday.

Western Heights High School principal Jim Gemmell said the call was answered by a receptionist, who told him the message seemed to be recorded and was playing on a loop.

He said it was a shock but his mind immediately turned to news of the previous calls, "but of course there is always that risk".

"We always put the safety of students and staff first."

The school was in lockdown for about two hours while all its buildings were checked by police. Mr Gemmell said police had advised students to stay away from buildings, and after-school trainings were cancelled.

Outside, a group of waiting parents became increasingly frustrated at not being allowed to pick up their children.

"We understand the frustration, but hope they understand we have the interest of all students at heart," Mr Gemmell said.

Letitia Banks said she had gone to the school to drop off lunch to her son and heard a siren going off.

She said she had been feeling pretty anxious and thought parents should have been notified straight away.

Elizabeth Pilaar was told about the threat when she arrived to pick up her daughter for a violin lesson.

"I feel angry that someone would threaten school kids and am obviously concerned for their safety," she said, as she waited for the school to be given the all-clear.

She said although she would like to take her daughter away, she was comfortable with how the situation was handled.

Mr Gemmell said school would go ahead as normal tomorrow.

"We will thank students and staff ... for their patience and how they dealt with the situation. The students were excellent."

Rotorua police directed media inquiries to national police headquarters.

In a statement, New Zealand Police said they had today "responded to calls of a concerning nature directed to 27 schools throughout the country". The calls were of an "automated nature".

"The calls may be linked to those received [Wednesday] which initial inquiries indicated have originated overseas," it said. "We should make it clear that while such calls may display a New Zealand number, they can still be originating from overseas."

It said similar incidents had occurred in the United States, and more recently Australia.

"Until police can rule out any actual threat, we must take all these calls seriously and respond accordingly.

"The safety of students, teachers and schools are our priority.

"We understand the fear and concern that this will cause, however we reiterate that so far we have not established any actual threat to any school."