Emergency services were called to a property on Elizabeth Drive in Vincentia, 200km from Sydney, at about 11.30pm.
About 40 people, all of them teenagers, had been standing on a balcony at the home when it suddenly collapsed.
It is understood the large group of teenagers had been having a party at the beachside home when the incident occurred.
Four people, three teenage girls and one teenage boy, were injured and were treated at the scene.
Paramedics took them to hospital for treatment of minor back and leg injuries.
