Residents from 51 units in a brand-new Sydney high rise were still unable to return to their homes on Wednesday following the block’s evacuation on Christmas Eve when cracks appeared on the 10th floor.

Emergency services have said there are no longer fears the Opal Tower could collapse, after loud cracking was heard by residents in Sydney’s Olympic Park on Monday afternoon.

About 300 people were evacuated on Christmas Eve, but Fire and Rescue New South Wales said on Tuesday two-thirds of the building’s 392 units were safe.

Fire trucks in front of Opal Tower during an inspection by firefighters and engineers early on Christmas Day. Photograph: Saeed Khan/AFP/Getty Images

The superintendent of Fire and Rescue New South Wales, Adam Dewberry, did not 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,” Dewberry said on Tuesday afternoon.

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.”

Dave, a resident of Opal Tower, told Guardian Australia his apartment was one of the 51 still affected.

“My apartment and my neighbour had notices on the doors that said you can’t actually live in here,” he said. “Nobody else was affected, it was just that corner.

Opal Tower resident Dave’s apartment has been declared unsafe after cracking was found in an internal support wall on the 10th floor

“Yesterday at about two o’clock I left my place to go to my parents’ place for Christmas. When I arrived, I got a text from a friend saying ‘Hey are you OK?’ She has a friend that is a couple of floors above me who said they got evacuated.

“I woke up at about six this morning and drove over there. There was a security guard there, I explained my situation, he said ‘OK you can go up and get your stuff, you only have five minutes.’”

Dave said he had lived in Opal Tower since the start of August, after the building was opened earlier this year. He said the tower was 80% to 90% occupied, based on his rough estimate.

Signs were left on the doors of some apartments, warning returning residents their units were unsafe

“The building strata management have been providing updates via email – not as detailed as I would like – but given the circumstances they are doing well. The builders and the developers are where my frustrations are at.”

Opal Tower, built by developer Ecove, was completed this year. Authorities said there had been no previous reports of cracking in the building.

A spokesman for Ecove said the company was “deeply sympathetic” for the concern and anxiety residents had been put through.

In an email seen by Guardian Australia, Waterpoint Management told residents the fault affected 35 floors of the building.

“This is due to a fault in a support wall in the building affecting floors ground to 35 on your side of the building,” it said. “Costs for places to stay will be attempted to be recovered by the responsible parties.”

On Tuesday morning Dewberry said people had first noticed the issue because the door jambs of the building shifted and residents couldn’t open their doors.

Police evacuated the building, including 55 children from its childcare centre, and a 1km exclusion zone was put in place.

On Monday, NSW police said the building had shifted between one and two millimetres – but there had been no further movement detected.

Police check the unit numbers of the residents after an inspection of the Opal Tower to determine whether it is safe for them to return. Photograph: Saeed Khan/AFP/Getty Images

Dewberry told reporters earlier on Tuesday that not all of the 51 units had been occupied. Those affected were also allowed to return briefly to collect some personal items.

“There were some pets and Christmas presents up there,” he said. “Obviously, it’s very difficult for the occupants at that time, being relocated on Christmas Eve. It’s an unfortunate event, however their safety was our first concern.

“They were relocated to the showgrounds, and throughout the night a lot of them found their own accommodation, through family and friends. Anyone who was left over was then assisted with temporary accommodation.”

He said he could not comment on any causes, but a local exclusion zone remained in the area of Opal Tower but all roads and public transport had returned to normal.

With Australian Associated Press