Exclusion zones have been set up around the troubled Mascot Towers apartment building in Sydney after new cracks were discovered in a brick facade.

Key points: The building had to be evacuated in June last year when cracking was discovered

The building had to be evacuated in June last year when cracking was discovered There are now concerns bricks could fall out of the facade

There are now concerns bricks could fall out of the facade Shops around the building have been closed

Engineers engaged by the building's Owners Corporation discovered the cracks yesterday and assessed there was a "small chance" of the bricks falling.

A confidential memo, obtained by the ABC, was sent by the engineers to the building's owners last night, explaining what was found.

The memo said there was a potential absence of cavity ties, used to anchor bricks to a structure, which meant the "risk of dislodgement cannot be excluded".

"It appears to the experts that the brickwork itself is taking unintended loads which is causing the cracking of that brickwork," the memo said.

"It was previously assumed that the brickwork facade would be safe from dislodgement based on the presence of cavity ties.

"Investigations have not been able to confirm tie-backs to the structure and so the risk of dislodgement cannot be excluded."

It said an "urgent risk assessment" was convened because the engineers considered falling bricks a "high risk".

The Owners Corporation notified SafeWork NSW, The NSW Building Commissioner, Fire & Rescue NSW (FRNSW) and Bayside Council last night.

Mascot Towers was evacuated in June after the first cracks were discovered. ( AAP: Bianca De Marchi )

Crews from FRNSW were called to the building just after 10:00pm and an exclusion area was taped off.

A security guard was stationed at the building overnight and a spokesman for FRNSW said more substantial hoardings would be put up today.

The ABC understands the cracks are limited to the building's facade.

In its memo, the Owners Corporation said it expected remediation works to take four to six weeks to complete.

It is not the first time cracks have been reported at the building.

Residents were forced to evacuate last June when substantial cracks were discovered in the building's primary support structure.

The repair cost was estimated to be at least $10 million.

In October, residents were advised those cracks had widened and new cracking had developed which required urgent remediation to avoid a "structural failure".

The NSW Building Commissioner David Chandler said he wasn't alarmed by the new cracks and did not suspect that there was "anything too calamitous than what we already know".

Mr Chandler said reports on how repairs were going had been positive, and bigger structural problems were unlikely.

"I'm told by independent experts that the building has reacted very well to the stabilisation of the slope of the basement, so they're not reporting anything of concern to me," he said.