Engineers engaged by the NSW Government say Sydney's Opal Tower building, which was evacuated after a crack formed in concrete, has "a number of design and construction issues".

Key points: Residents were evacuated before Christmas over fears a crack in concrete might have meant the building was unsafe

Residents were evacuated before Christmas over fears a crack in concrete might have meant the building was unsafe They are still being put up in hotels and are unsure when they will be able to return

They are still being put up in hotels and are unsure when they will be able to return Engineers say a number of design and construction issues have been identified

There were fears the 34-storey building was structurally unsafe when the defect was found on Christmas Eve.

Some residents were allowed back in before being ordered out again days later.

The builder behind the project, Icon, has launched an investigation alongside the State Government's own probe, which is being led by University of NSW dean of engineering Mark Hoffman and Emeritus Professor at the University of Newcastle's School of Engineering John Carter

The pair said they were still waiting for key information to complete their investigation.

"We can say from our initial assessment there is no evidence of any issues with the foundations of the building, though we believe that there are a number of design and construction issues that require further investigation," they said in a statement.

Building damage on the exterior of Opal Tower at Sydney Olympic Park. ( Supplied )

The two will now focus their attention on those specific issues to determine the overall problem.

They said they hoped to make another statement by the end of next week.

Housing Minister Anthony Roberts said any decision regarding when residents could return to the building is one for the owners.

Residents have previously been warned the investigation may take 10 days but there was no guarantee when they would be allowed to return.

They have been put up in nearby hotels and compensated.

Ehsan Jahanandish has lived in the building since September and said he was concerned.

He was also concerned about a plan to let people back in the building as early as on Monday.

"They don't know the cause of the problem, how do they want to risk people's safety again and put them back in the building?" he said.

Sorry, this video has expired About 3,000 people were evacuated from the 34-storey building and surrounding areas. (Photo: AAP)

NSW Premier Gladys Berejiklian said the Government empathised with Opal Tower residents.

Following the Opal Tower issues, the NSW Government announced a crackdown on dodgy building certifiers.

Among the new measures were a large compliance drive which would see up to 30 per cent of certification work audited every year.

Certifiers who are found to be corrupt or to have negligently signed off on an unsafe building will also be booted from the industry.