Fifty-one units in Sydney high rise unsafe

Some residents have returned to the Sydney Olympic Park high rise that authorities feared was at risk of collapse but it will be a grim Christmas for other residents after 51 units were deemed unsafe.

Engineers and the building's managers remained at the 38-storey building overnight but emergency services have since left after declaring there were no longer concerns of the tower collapsing.

However it is not yet known how long residents of the affected units will be without their homes.

Fire & Rescue NSW Superintendent Adam Dewberry doesn't expect those units to be declared safe any time soon.

"It's not as if it's going to be tomorrow or the next day because there needs to be an element of work done, now it's just how quickly that work can be done by the engineers to get in and get a really good thorough assessment," Supt Dewberry told AAP.

He said authorities weren't yet aware of how many people were affected.

"The actual number we're not sure of, because of the time of year, because of the evacuation and people have sorted out their own accommodation and got themselves moved on."

Emergency services were called to the site on Monday afternoon following reports residents had heard and seen cracking in a wall throughout the morning.

About 300 people from more than 150 occupied units in the Opal Tower were evacuated amidst fears the brand-new high rise could collapse.

Police also evacuated neighbouring buildings and 55 children from a nearby childcare centre and put in place a one-kilometre exclusion zone.

Fire & Rescue NSW Urban Search and Rescue officers and specialist engineers, as well as the building's designers and managers, worked late into the night to assess the building.

Just after midnight on Christmas Day, residents of the units that had been declared safe were allowed to return home.

Residents of the 51 units that were considered unsafe were escorted back to gather their belongings. They've made their own arrangements for accommodation, according to NSW Police.

Supt Dewberry said the strata manager would be working with the insurer and builders to fix the issue.

He said a local exclusion zone remained in the area of the 392-apartment Opal Tower but all roads and public transport had returned to normal.

A spokesman for the building developer Ecove Group said they were "deeply sympathetic" for the concern and anxiety residents had been put through.