Residents in a newly opened apartment building saw and heard a loud crack before thousands were evacuated from the tower in Sydney's west.
Police confirmed the 34-storey Opal Tower had moved "1 millimetre to 2 millimetres".
About 3,000 people were evacuated from the Sydney Olympic Park building and surrounding areas after the alarm was raised about 2:45pm on Christmas Eve.
Many residents face an uncertain Christmas Eve night, with experts entering the building — which only opened four months ago — to determine the extent of the damage.
An emergency operation is still underway and police have closed the nearby train station and several roads.
NSW Fire and Rescue acting superintendent Greg Wright said the crack was on level 10 of the building — and although it had moved, it had not moved since this morning.
"[We're using] laser building movement monitors [there now] to determine if there's any movement," he said.
One resident told Nine News he heard a "big bang" like something "snapped" inside the building.
Residents have not yet been told when they will be allowed back in the building, if at all, and many are preparing to spend the rest of Christmas Eve at an evacuation centre in central Sydney at the The Royal Hall of Industries.
NSW Fire and Rescue said it had specialist equipment at the scene to monitor any movement and police have set up an exclusion zone amid fears the building has serious structural damage.
The doors of the building were jammed and the police had to break through to help residents out.
Police were at the scene carrying heavy-duty equipment to knock down doors.
One Opal Tower resident said he was concerned.
"A few days ago the doors looked different. Like they couldn't close the doors properly. And you do feel it [movement] sometimes when there's strong wind."
Sydney Trains have suspended services at Olympic Park.
Opal Tower opened this year and contains 392 apartments.
It is part of a mixed-used development by privately owned development company Ecove and also includes a single-storey childcare centre.
Ecove spokesman Mark Abraham said the building was assessed by the "relevant authorities".
"We are aware of the concerns that have arisen with the Opal building and confirm that the builder has been notified," Mr Abraham said.
It was designed by Australian architecture firm Bates Smart.
Ecove is also behind the nearby "Boomerang building" and a number of other high-rise buildings in Sydney.
