Dozens of families in Sydney's cracked Opal Tower have been told they must wait weeks after repairs are complete before they can move back as engineering firms finally agree on what repairs must be made to the building.

The tower’s builder Icon has agreed to keep funding accommodation for displaced residents but will cut off payments for food in less than a week.

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

Icon, in an update posted on their website on Tuesday, said the design for “structural remediation” had been finalised by engineering firm WSP and submitted to body corporate engineers Cardno for agreement.

“The remedial works will be carried out on Levels 4, 10 and 16 which involve strengthening the affected upturn beam via the introduction of a column or wall to one/both sides of the beam,” Icon wrote.

Residents told AAP the repairs were described to them as using a “sandwich method” to strengthen beams in the structure.

About 170 apartments have already been deemed fit for reoccupation but the remaining 224 units were given a new schedule for reoccupation on Tuesday.

Apartments not affected by the repair works, 50 in total, will be free for reoccupation two weeks after Cardno signs off on the plans and strengthening works are completed on level 16.

A further 140 apartments are awaiting approval for engineers and are expected to be occupied in four to six weeks after Cardno and WSP reach an agreement.

A final 34 apartments which are affected by the repair works will be slowly opened up to residents between three and five weeks following the completion of repairs.

There is no indication when the repairs will be completed.

Further, Icon said it would not pay for food for displaced residents after February 16.

The company will continue to fund accommodation now in longer-term increments as repairs are carried out.

Families are expected to be placed in serviced apartments during their stay.

One resident told AAP on Tuesday more than 200 people had signed on to a class-action lawsuit after their property was rendered unsafe for occupation.