Chief fire officer Kevin Evans said it was a situation that called for a "precautionary approach".
Initially reported as an explosion by the fire communications team, Mr Evans said it was more "a reaction".
"It sounded dramatic [to begin with] but turned out to be nothing too extraordinary," he said.
"There were little puffs of chlorine and the odd pop and crackle but certainly not an explosion."
Those inside a nearby house were evacuated as the reaction continued for more than an hour.
A 100-metre emergency evacuation zone also remained in place at the deserted school yesterday afternoon.
"We did take a precautionary approach and wanted to make sure no one was in the vicinity," Mr Evans said.
"But it wasn't a very big event at all. It just meant the swimming pool got an extra dose of chlorine."
Ruawai College assistant principal and school health and safety officer Lance Panoho was onsite yesterday looking into what had happened and assessing the condition of the pool.
He was unable to comment, other than to say he was disappointed the incident had forced him off the golf course for the day.
