A 12-year-old boy who impaled himself on a fence at an Auckland school this morning was attached to the structure for 25 minutes before he was cut free.

Emergency services were called to Waimahia Intermediate School in Clendon Park, south Auckland at 8.34am, where the boy had tried to climb a 1.8m high fence but slipped and impaled his hand.

Manurewa Fire Station Officer Roy Harris said the boy was distressed, but was given pain relief as soon as 10 minutes after he impaled himself.

"He managed to put the spike of the fence through the fleshy part of his hand. We had to remove part of the fence to get him off to hospital to have it removed.

"He was a bit distressed when we first got there, but he was given some pain relief and we were able to cut him free."

Mr Harris said the Fire Service's Manurewa crew and the specialist cutting equipment from Papatoetoe arrived about 8.42am and the boy was taken to hospital about 9am.

St John paramedics were also at the scene and administered pain relief quickly, he said.

A St John spokesman said the boy was transported in a moderate condition transported to Middlemore Hospital.

A school spokeswoman said the boy was "fine" and on his way to hospital for treatment.

"The boy is fine, absolutely fine. He is off to the hospital and the professionals are taking care of him."

She said some students witnessed the accident and were being looked after by staff.