The engineering firm investigating what caused a large crack to develop on a concrete panel in Sydney's Opal Tower has discovered more damage in the building.

Key points: Residents in the vicinity of the crack could be displaced for months

Residents in the vicinity of the crack could be displaced for months Engineering firm WSP said the building was "structurally sound overall"

Engineering firm WSP said the building was "structurally sound overall" Precautionary propping was installed at the site of the newly discovered damage

The 34-storey building at Sydney Olympic Park was first evacuated on Christmas Eve — and again three days later — after a crack was found on level 10.

Engineers from WSP said they found two areas with "lesser damage" on level four but said apartment complex was "structurally sound overall".

"As a precautionary measure, propping is being installed to support level four and this will be completed by Icon Co by the end of today," WSP said in a statement.

"All involved parties are working cooperatively with the Government's independent engineers, who are investigating the cause of the damage.

"There will be further investigative testing in relevant parts of the building."

Icon Co is the construction company that built the tower.

Cracks were found in a pre-cast concrete panel inside the building on Christmas Eve. ( Supplied: Weibo )

In the statement, WSP warned residents on level four would not be able to return to their apartments until repairs were complete.

That could mean they are displaced for months.

However, WSP said it supported a progressive reoccupation of the majority of units in Opal Tower "subject to agreed safety processes".

It was unclear how long that would take. Today is the fourth day of an investigation in to the apartment complex which the builder, Icon, predicted would take 10 days.

In a statement released on New Year's Day, developer Ecove said it was confident there was no issue with the foundations of Opal Tower and what occurred on Christmas Eve.

"Today, our focus remains on staying in touch with Icon [the builder] to ensure the welfare of our residents and to return them to their homes as soon as possible," Ecove said.

The NSW Government last week launched an investigation into the tower's structural integrity, appointing two university professors to write a report which will be made public.

Last week the Government announced a crackdown on "dodgy building certifiers" through a large compliance operation which would see 25 to 30 per cent of certification work audited every year, undertaken in a "strike-force-style approach".

A worker on a balcony at Opal Tower ( ABC News: Lily Mayers )

However NSW Labor leader Michael Daley accused the Government of being missing in action over the issue and leaving residents without appropriate support.

Speaking for the first time on the issue after releasing a written statement last week, NSW Premier Gladys Berejiklian said the Government was doing "everything it can" to get to the root cause of what happened.

"We've got experts in the field supporting those engineers," she said. "We were active from the first day.

"To have it happen at this time of the year would just be awful, and I really empathise with all residents.

"But we need to deal with the facts [so] we can make sure all buildings are safe."