Police, fire officials and paramedics were called to a home on Green Village Lane in Dartmouth around 1:30 a.m.
Staff Sgt. Bill Morris said numerous people were on the deck when it collapsed nearly four metres to the ground.
About 15 people were taken to hospital with injuries ranging from cuts to fractures.
Morris said four or five people have serious injuries.
"Originally we believed (the injuries) were life-threatening, but we've been told that they are not life-threatening," said Morris.
The cause of the accident is under investigation, he said.
"We're going to turn the investigation over to our general investigation section to let them continue with it, probably in conjunction with the (Halifax Regional Municipality) and their building inspection people," said Morris.
The deck was removed from the yard on Sunday.