Fire and Emergency New Zealand (Fenz) said they received calls about the blaze about 12.15pm.
The Silverdale Volunteer Fire Brigade, which was nearly half an hour away from the college, attended even though the closest fire station, Birkenhead, is four minutes away.
The fire started during today's one hour strike by the Professional Firefighters Union. Fenz said during that hour, it was relying on volunteer brigades.
There were now other brigades on the scene.
Smoke could be seen from the Harbour Bridge billowing from the school's sports pavillion, a large wooden hall with a high pointed roof.
On social media, a school spokesperson said: "There is an active fire at Northcote College in the sports pavilion. The fire service is here.
"All students have been evacuated to the other end of the school and are safe."
On its website, a spokesperson said the school would be closing for the day at 2pm.
"Some students may not have their bag because of the evacuation. We are asking students to go home, if they can, without their bag. Students who are unable to get home or need to call home are meeting in the hall and will be supported by staff.
"An email to all students and whānau with further information will be sent later today."
It took the volunteer Silverdale crew about 17 minutes to arrive at the school.
It appeared they had been close to the area for another job.
The first career firefighters arrived at 1.13pm, he said.
Communications call centre staff were also on strike for the hour, with managers taking 111 calls and cooridinating call-outs.
Just last month another large fire broke out during strike action.
A building in Pakuranga was completely destroyed by fire and a person was seriously hurt.
At the time, Pakuranga MP Simeon Brown said he was "angry" on behalf of those impacted by the fire due to it happening during the strike.
"Union action that delays a response to an emergency is quite frankly reckless and the union needs to put a stop to these reckless strikes which endanger lives, homes, and businesses."
New Zealand Professional Firefighters Union secretary Wattie Watson said contingencies were meant to be put in place during the strike.
Northcote MP Dan Bidois thanked local police and firefighters from across Auckland for the quick response - and to the school staff for an "orderly fire evacuation".
"Glad everyone is safe."
Bidois said the building on fire was used to store gym equipment.
On social media, North Shore councillor Richard Hills said it was "so sad" to see another fire at the school.
"It will be hugely upsetting to students, staff and school whānau, especially as they're just getting back to normal, after the previous fire, and recent opening of new and upgraded buildings post construction.
"The fire service are there and thankfully all students have been evacuated to the other end of the school and are safe. The fire is very much still active."
Hills said it was likely to cause traffic delays in surrounding areas and urged people to stay away if they didn't need to be there.
