THREE women have life-threatening injuries and another 27 suffered lesser injuries after a balcony collapsed under the weight of 70 people attending an end-of-school party in Brisbane.

The group, comprising mostly mothers celebrating their sons' Year 12 graduation from the Anglican Church Grammar School, were on the balcony of a renovated Queenslander-style home in Ascot when it collapsed.

Firefighters freed three people who were trapped under the debris.

A total of 27 people were taken to various Brisbane hospitals, three with life-threatening injuries.

Their children were attending a fun day on the Gold Coast when they found out about the collapse, mostly by text message.

School principal Jonathan Hensman, who was uninjured, was about a metre away from the balcony when it collapsed.

Talking to AAP at the scene, with his left shoulder covered in another person's blood, he described the scene as a "mass of bodies laying amongst smashed glass".

"It's a very tragic event to happen on a day we were meant to be celebrating," he said.

"It was just a mess."

One of the mothers, Anne Chater, said the collapse took everyone by surprise.

"I was extremely lucky. I was at one end of the balcony when the other end collapsed," she said.

"It just collapsed, it just went thump.

"I couldn't help anyone."

Kathy Parer, whose daughter was at the party, said she responded to an urgent phone call.

"I got a phone call from my daughter in tears and I raced here as quick as I could," she said.

"I was just stunned."

She said it was a tight-knit group and they would come through the tragedy.

"All the girls have stuck together really well," she said.

"It's helped that they have all known each other and helped each other get through it.

"It's a pity it's happened on such a great day."