'Cracked' Opal Tower caused by design, construction flaws: independent report

The NSW government has vowed to implement the findings of an independent report into the 'cracked' Opal Tower.

Residents of Sydney's Opal Tower were evacuated on Christmas Eve after cracks were spotted. Almost a year later, they're allowed to return. Source: AAP

Critical support beams in Sydney's Opal Tower were left susceptible to "bursting" because they were under-designed and - in parts - made from lower-strength concrete, an independent report has found.

The newly-built block in Sydney Olympic Park was evacuated on Christmas Eve after cracks were found in the building, sparking fears it could collapse.

A NSW government-commissioned report, released publicly on Friday, found that while the building was structurally sound overall, there were a number of structural and construction issues responsible for the damage.

“We will be working through the detail of today’s report with our new Building Commissioner to make sure that when homeowners are handed over the keys to a new building, it is safe, structurally sound and free from major defects," Minister for Better Regulation Matt Kean said.

The engineers' report found horizontal support beams in the building were of inferior strength and were not compliant, while the decision to only partially grout between the beams and panels added to the problem.

"We found some of the as-constructed hob beams and panel assemblies were under-designed according to the National Construction Code and Australian Standards," professors Mark Hoffman, John Carter and Stephen Foster said.

"This left the hob beams susceptible to failure by shear compression and bursting."

The causes of damage at Sydney's Opal Tower as summarised in the final report. Source: Unisearch

Testing also showed lower-strength concrete was used in some hobs on level four, which "likely precipitated the observed major damage".

The engineers' report recommended creating databases of registered engineers and NSW building certifications, along with a building structure review board to monitor known property design flaws and shape future building codes.

“This has been a very tough time for residents and owners at Opal Tower and I first want to thank them for their patience and support in allowing the independent investigation team to prepare this detailed final report," said Minister for Planning and Housing Anthony Roberts.

“I also want to thank Professors Hoffman, Carter and Foster for their significant efforts to complete this report in a timely and professional manner,” he said

About 221 of the building's 392 units remain unoccupied.