The NSW Government's investigation into damage at Sydney's Opal Tower has found the cracked apartment block needs "significant rectification works".

Key points: Engineers say there are damaged hob beams that require repair and strengthening

Engineers say there are damaged hob beams that require repair and strengthening The 392 apartments were evacuated on Christmas Eve when defects were found and residents heard cracks

The 392 apartments were evacuated on Christmas Eve when defects were found and residents heard cracks Residents say they don't want to live in a construction site and have described buying into the building as an "Australian nightmare"

The building, which opened last August, was evacuated on Christmas Eve and again three days later after a large crack was found on level 10.

An interim report from Professors Mark Hoffman, John Carter and Stephen Foster released on Tuesday found several factors had likely contributed to the damage.

In a statement they said the building was "structurally sound" but that "significant rectification works" were required to repair and strengthen some damaged beams.

The trio said while the "probable cause" was narrowed down to "localised structural design and construction issues", more information was needed to make a conclusion.

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

Many Opal Tower residents are yet to be allowed to return to their homes and NSW Housing Minister Anthony Roberts said that was a matter between the residents and the builder.

"I completely understand the residents' reluctance in some cases to move back in to what is essentially going to be, for some time, a building site," he said.

The engineers behind Opal Tower, WSP, told the ABC on Tuesday remediation work would "fully restore the building".

"WSP believes it is appropriate for most residents to return to their units," WSP Australian and NZ chief executive Guy Templeton said.

"There are significant parts of the building which are safe."

'An Australian nightmare'

Resident Shady Eskander said a meeting with about 300 owners was held on Saturday and the group overwhelmingly decided not to return until the building was restored to the state it was in when they purchased.

"No owner deserves to have this happen to them anywhere in Australia," he said.

"I can say, on behalf of all the owners, people were buying into the Australian dream and unfortunately this has become an Australian nightmare for us."

Opal Tower resident Shady Eskander said owners don't want to return to a construction site. ( ABC News )

Professor Hoffman said the document outlined what needed to be investigated and checked before residents were allowed to return, emphasising that it was an interim report.

"We've been able to make a lot of progress but there's still more work to go," he said.

Mr Roberts was not able to say how much the fiasco would cost.

"I can assure you that there is not just legislation in place, but there are certainly issues that will be played out between the developer as well as the builder with respect to this," he said.

"I prefer not to comment on that any further, it is not in any way part of our jurisdiction."