The woman was in the intensive care unit of the Royal Brisbane Hospital when she passed away, a police spokesman said.

The balcony on the house in Upper Lancaster Road collapsed during an end of Year 12 mothers' luncheon about 1pm.

As many as 70 women were attending the annual function when the balcony plunged to the ground, injuring 25 people and trapping several.

Operating theatres were immediately placed on stand-by yesterday afternoon, ready to receive casualties, and intensive care beds were reserved for those requiring immediate and post-operative treatment, the spokeswoman said.

Two other women were taken to the Princess Alexandra Hospital with serious injuries and 12 others described as "walking wounded" were taken to the Mater Hospital for observation and treatment.

Police, fire and ambulance crews which swarmed the scene this afternoon cordoned off Upper Lancaster Road and medicos set up a makeshift clinic underneath a tent on the front lawn of the replica Queenslander house to treat the injured.

Witnesses reported hearing a "crack" before the balcony gave way.

One shaken witness, known only as Millie, said the lunch had been for mothers of Year 12 students at Brisbane Anglican Church Grammar School.

Another, whose daughter attended the luncheon and was injured in the dramatic fall, said she had been notified by telephone.

"I received a teary phone call... she's been taken to the Mater and it seems she's done some harm to her leg," Kathy Parer told reporters.

"They're all terribly shaken but they're coping very well."

A resident in the street said it appeared just the flooring of the three-metre by five-metre balcony gave way, leaving the structure intact.

"The rest of the building is intact, it's basically just the flooring of the balcony that has given way," said the man, who did not want to be named.

"It's sounded like a crash you know, I thought initially it might have been a vehicle accident.

"There just seems to be a lot of people in shock."

The cause of the collapse is not yet known.

It is believed the incident occurred at the home of school captain Jack Biggs, whose father Tim Biggs is an accountant with city firm Deloittes.

Jack Biggs declined to comment when contacted by brisbanetimes.com.au earlier.

One Year 12 student at the school, who requested anonymity, said his mother was at the function, but had not been injured.

"We are distressed, but we're staying positive," he said.

"It could have been a lot worse."

A telephone hotline has been set up for anyone worried about friends or family.

The number is 1800 100 188.

with Georgina Robinson