Engineers and firefighters were preparing to enter a 36-storey high-rise Olympic Park building that was at risk of collapsing after “cracking” on the 10th floor shifted the structure.

About 3,000 people had been evacuated from the Opal Tower building and surrounds on Christmas Eve, while a 1km exclusion zone was in place.

Fire and Rescue NSW crews and Urban Search and Rescue teams were about to enter the 10th floor to investigate the source of the cracking, Fire and Rescue acting inspector Greg Wright said on Monday evening.

“It’s not going to be done in minutes – hopefully it won’t take longer than hours,” Wright told reporters in Sydney.

The building had shifted between one and two millimetres as a result of the issue, police detective Superintendent Philip Rogerson said. But there had been no further movement detected on laser monitors deployed by emergency services.

Residents from the Opal Tower and nearby properties were not being allowed back to their homes. Instead, they were being sent to an evacuation centre until the building was deemed safe for re-entry. There was no expected timeline on when that would be.

“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,” Wright said.

Police evacuated 55 children from a nearby childcare centre and released them to their families.

“The No 1 priority for us is the safety of people – both the people inside the building and people in surrounding areas,” Rogerson said.

There had been no prior reports of cracks at the building until Monday, authorities confirmed.

Earlier on Monday, Wright said water and gas had been shut off and the tower was being isolated from the power grid.

“Fire and Rescue have deployed laser building monitoring equipment to check for movement,” he said.

Sydney Olympic Park: People evacuated from Opal Tower on Australia Avenue. Firefighters say a crack has appeared in the high rise apartment block. Specialist rescue trucks with building movement sensors on them are now on the scene and more emergency resources are on the way. pic.twitter.com/78IlQ4Wqdd — 7 News Sydney (@7NewsSydney) December 24, 2018

Meriton, the operator of the nearby Botania building told residents in an email that as “there is a potential for the tower to collapse, a 1km radius has been evacuated”.

The website for the Opal Tower says the building has 392 apartments, commercial spaces, a childcare centre and community centre. It cost $165m to build.

Trains have been stopped as a result of the operation, the Transport Management Centre confirmed on Monday.

A BBL cricket match between Sydney Thunder and the Sydney Sixers was expected to go ahead at the nearby Spotless Stadium with fans being instructed to travel to the venue by bus.

Fans were instructed to enter the stadium via Hill road.

Australia Avenue was been closed in both directions.