Balmoral Fire senior station officer Mark Griffin said heavy traffic meant it took some time for a fire crew to arrive.
"Basically it was just a weather squall that came through and some of the branches from the trees tangled in a couple of power lines, and they went bang and broke."
The fire service was called to the scene at 3.15pm but heavy traffic delayed the crew's arrival, Griffin said.
"The teachers and staff were really good. They coned the area off and were directing traffic ... they protected the scene very well until we could get there."
The lines were treated as live until Vector officials tested them, and found that "thankfully" the power had been cut, Griffin said.
"In this case it would have been because as soon as the power lines broke, the power was isolated immediately. But until they've tested them, we don't know that and they don't know that. So everybody was lucky in that respect."
A witness told Griffin there was a big bang as a line got tangled in branches from an elderly overhanging tree.
"There was an explosion, a bang and a flash of light."
A Vector spokesman said the incident caused a power cut in Sandringham and Mt Albert.
"The exact cause hasn't been determined but it's likely to be weather-related," he said. Power was expected to be fully restored by 6.30pm.
"We recommend customers download our free Vector Outage app to get up-to-date information on outages across our network."
