A family was lucky not to have sustained more serious injuries when the balcony they were eating dinner on gave way and they plunged up to six metres onto concrete, their rescuers say.
One woman is expected to undergo surgery at the Royal North Shore Hospital on Thursday but nine others have since been released from hospital after the balcony at a duplex in Lane Cove collapsed about 10.15pm on Wednesday.
Inspector Jeff Bell, the duty officer from the North Shore local area command, said a man and his fiance had gathered their siblings for a family dinner at one of their parents' place on Wednesday night when the accident occurred.
The group of 12 people had just finished eating dinner and had stood up to head back inside when the timber balcony suddenly gave way.
The 12 people and the furniture, including table and chairs, slid off the balcony, which was left hanging from the side of the duplex.
"They got up to go inside and the part of the balcony that connects to the house basically disconnected from the house and then went fairly vertically. They all just slid off it, so to speak," Inspector Bell said.
"The first floor had about a one-metre wide patio on it. Some of them fell onto that, which stopped the fall, and half of them fell just straight down into the garage section, another three metres further."
He estimated five of the group fell the full six metres into the garage.
"Considering the distance they fell and the potential for injury, falling onto a hard surface, and various bits of furniture fell with them, table and chairs and all the rest of it, it was quite amazing the injuries weren't worse," he said.
Eight ambulances and a helicopter were called to the duplex on the Pacific Highway and they joined police in a delicate 1.5-hour operation to rescue the casualties and secure the scene.
"Some of them had to be stabilised with neck braces because of the potential for spinal injury," Inspector Bell said.
"The ones that were on the first floor level on the patio, it was quite a narrow stairway and they had to lift them over their heads. As far as things go, the rescue was surprisingly smoothly," he said.
Ten people were taken to Royal North Shore Hospital.
Inspector Bell said nine had since been released, but a 31-year-old woman remained in the intensive care unit and was expected to undergo surgery on Thursday for a number of fractures.
Neighbours reported hearing a large crash from 300 metres away when the balcony collapsed.
Inspector Bell said the duplex was an older, brick building that appeared to have been renovated. The deck was added eight years ago.
"It's an eight-year-old deck which had stood the test of time, until now. What caused this will have to be investigated," he said.
Engineers are expected to begin examining the site on Thursday morning.