Almost half of the apartments in Sydney's troubled Opal Tower are still vacant, nearly four months after the building was first evacuated on Christmas Eve.

Key points: Only 90 of the 392 apartments have been reoccupied since late February

Only 90 of the 392 apartments have been reoccupied since late February Resettlement was delayed due to pending engineering advice

Resettlement was delayed due to pending engineering advice Many residents had vowed to never return after they were evacuated

This means only 90 apartments have been resettled in the 36-storey Olympic Park building since late February — 169 of the tower's 392 units remain empty.

A spokesperson for the tower's builder Icon Co told the ABC it was recently given permission from the owner's corporation — a collective body comprised of the tower's residents — to proceed with rectification works to common areas of the tower.

The owners corporation said it had taken so long because it was waiting on engineering advice before giving Icon the go ahead.

"It's been a long process, unfortunately engineers don't work as quickly as we'd hoped they would," said the chairman, Shady Eskander.

The owner's corporation on Friday gave formal consent for the works to commence.

Shady Eskander is the chairman of the tower's owners corporation. ( ABC News )

Icon had hoped to begin resettling empty apartments over the coming weeks while repairs were underway.

However, Mr Eskander said that was unlikely.

"The affected apartments that are unable to return, it will probably be a couple of months," he said.

The apartment complex was first evacuated just before Christmas after residents heard loud cracking noises — damage was later found on several levels.

The incident prompted the State Government to commission a report on the building, spearheaded by three of NSW's leading engineers.

It found parts of the tower were constructed using "lower-strength concrete" and that "under-designed" critical support beams had burst under extreme pressure.

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

Occupants were stranded in temporary accommodation after they were evacuated.

Mr Eskander said the owners corporation had negotiated an extended warranty on the Opal Tower, something that had "never been done before in Australia".

"Our number-one interest is their [the owners'] safety and wellbeing," he said.

"Investors who've lost tenants, we've also been able to ensure that Icon distribute the equivalent of their lease, their rent in their lease, to them until the rectification works are complete."