Engineers and firefighters are investigating what caused the 36-storey high-rise building to shift between one and two millimetres.
Emergency services were called to the Opal Tower on Monday afternoon following reports residents of the Sydney Olympic Park building had heard and seen cracking on the tenth floor throughout the morning.
The Opal Tower is seen at Olympic Park in Sydney on 28 December, 2017. Source: AAP
About 3,000 people have been evacuated from the building and surrounds, while a one kilometre exclusion zone is in place.
FRNSW crews and Urban Search and Rescue teams entered the 10th floor to investigate the source of the cracking, Fire and Rescue Acting Inspector Greg Wright said.
"It's not going to be done in minutes - hopefully it won't take longer than hours," Inspector Wright told reporters in Sydney at 7pm.
The building has shifted between one and two millimetres as a result of the issue, Police Detective Superintendent Philip Rogerson.
But there has been no further movement detected on laser monitors deployed by emergency services.
Residents from the Opal Tower and nearby properties are not being allowed back to their homes.
Instead, they are being sent to an evacuation centre until the building is deemed safe for re-entry.
There is no expected timeline on when that will be.
The Opal Tower is seen at Olympic Park in Sydney, on 14 January, 2018. Source: AAP
"We don't know (when that will be) until the engineers assess the building and find out what caused the issue - or if there's a major issue with the building," Inspector Wright said.
Police evacuated 55 children from a nearby childcare centre and released them to their families.
"The number one priority for us is the safety of people - both the people inside the building and people in surrounding areas," Detective Superintendent Rogerson said.
Police and fire officials conduct an operation to evacuate 3000 people from the surrounding area. Source: SBS
There have been no prior reports of cracks at the building until Monday, authorities confirmed.
The website for the Opal Tower says the building has 392 apartments, commercial spaces, a childcare centre and community centre.
It cost $165 million to build.
Trains have been diverted away from the building as a result of the operation and buses are being used to transport fans to a nearby BBL cricket match at Spotless Stadium.
Australia Ave has also been closed in both directions.
