A four-storey building has dramatically collapsed just minutes after workers found a massive crack in the wall.
Ten tradesmen were inside the commercial property on Alexander St in Crows Nest, on Sydney's lower north shore, working on an expansion.
About 3.25pm on Monday, they noticed large cracks in the wall and notified authorities before evacuating the building.
Within minutes of running for their lives, the building crumbled leaving a massive pile of rubble and rooms hanging spilling over the edge of walls.
Ten tradesmen were inside the building on Alexander St in Crows Nest, on Sydney's lower north shore, working on an expansion when it collapsed (pictured)
The building's manager Penny Flanagan quickly ran through all four levels telling everyone to get out.
'Windows started giving way so it was pretty clear it was a big problem,' she told Nine News.
'People are the most important thing to us.'
A worker who wished to remain anonymous said Ms Flanagan was a hero after warning everyone.
'We heard the crack and the building manager got into gear so quickly and took responsibility,' the woman told The Daily Telegraph.
'There was no time for laptops, or to take anything out - people were the priority - and if it wasn't for that that it could have ended very badly.
'She's the hero of the day.'
Part of the four storey building suddenly collapsed after workers noticed a huge crack in the wall (pictured)
Luckily no one was injured during the incident as an investigation is underway into what caused the building to collapse (pictured)
Horrific footage shared by Seven News shows neighbours watching the scene before the building collapsed.
Four fire crews attended the scene and established that the whole building would need to be demolished.
'That's four levels high of building material ... we've got one crushed car that's visible ... other parts of that wall have major cracks in them and are also under threat of falling own,' Rob Jansen from NSW Fire and Rescue said.
'The roof is still intact.'
An investigation into what caused the building to collapse is underway.