Most were allowed back inside after the building was declared safe. However, many remained in limbo as 51 apartments in the tower were off limits to residents days after the incident.

"The building is structurally sound and the temporary relocation is a precautionary measure to allow engineers to work around the clock to comprehensively investigate and remediate the site in the quickest time frame possible, without further disruption to residents," the builder, Icon, said in a statement.

Icon will pay for those affected to stay at nearby hotels.

"The damaged section of the building has now been reinforced as a precautionary measure while a team of engineers carries out the investigation.

"The thorough investigation is being led by engineering firm WSP, with assistance from senior engineers from Kajima Corporation, one of Japan's largest construction companies and majority owner of Icon."

Speculation and recriminations flowed over the cause of the damage, with some experts pointing to a broader pattern of cost-cutting within the building industry amid the city's development boom.

Meanwhile, NSW Planning Minister Anthony Roberts said two university professors who were structural engineering experts would urgently probe the circumstances surrounding the cracked panel.

Mr Roberts and Planning Department secretary Carolyn McNally on Thursday appointed Professor Mark Hoffman, dean of engineering at the University of NSW, and Professor John Carter, dean of engineering at the University of Newcastle, as specialist investigators to lead the inquiry.

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The terms of reference for the inquiry show the investigation would examine the "basis of the failure" and the immediate steps needed to ensure the safety of the building for its occupants.

They will also make a series of recommendations on what needs to happen to avoid similar incidents in the future.

The probe's findings will be made public.

“While the investigators undertake their work, the priority is to secure any displaced resident appropriate accommodation and support," Mr Roberts said in a statement.

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“I’ve been in contact with the developer and the builder in the last 24 hours and have stressed to them my expectation that these residents are looked after."

The project's developer, Ecove, has defended the "high-quality building". Ecove director Bassam Aflak said in a statement that Icon had "full liability on the design and construction of Opal Tower".

An Icon spokeswoman said the company accepted its responsibility.

Meanwhile, Mr Aflak hit back at suggestions the damaged tower pointed to a broader pattern in the building industry and said there had been "no cutting of corners" in the project.

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"It’s too early to leap to any conclusions, for instance, to speculate on wider building issues because of what appears to be one failed panel.

"The requirement from the Sydney Olympic Park Authority was that the building be high quality, and we have delivered on that.

"While this incident suggests otherwise, the contract with Icon specified that the building be of high quality Australian design and construction."

The Planning Department signed off on the tower block as a state significant development, which meant it bypassed local council approval, in June 2015. It was inspected by a private certifier.

The block opened to residents in August.