An 18-year-old apprentice has been crushed to death and another worker, 39, has been critically injured after scaffolding collapsed at a building site in Sydney’s north-west on Monday.

Formworker Christopher Cassaniti, who initially couldn’t be accessed by paramedics, was pronounced dead a few hours after the temporary structure fell at the Macquarie Park building site about 12.20pm, police say.

When Mr Cassaniti turned 18 last week, his mother Patrizia sung the praises of her “compassionate and shy, yet strong and forever helpful” son.

“We wish you all the best that the world can dish out and May God be forever in your heart and bless you with much joy, happiness and most importantly health,” she posted to Facebook.

MACQUARIE PARK | #FRNSW crews are assisting @NSWAmbulance with a delicate rescue operation at a construction site in Epping Rd. Two people are trapped under a large amount of fallen scaffolding. pic.twitter.com/Ff3QUTvZry

— Fire and Rescue NSW (@FRNSW) April 1, 2019

His friends posted tributes to Facebook on Monday.

“You were a fantastic bloke and had a heart of gold,” Benjamin Betros said.

“You have been my friend since the first day of kindergarten .. words can’t express how much this hurts,” Bella Mineo said.

His older workmate with lower body injuries was freed by Fire and Rescue NSW workers and taken in a critical condition to Royal North Shore Hospital.

Tonnes of steel, concrete and meshing were strewn over the site of the Lachlan’s Line apartment complex after the 15-to-17-metre scaffolding tower collapsed.

“The first (paramedics) to arrive at the scene found an absolute chaotic, emotionally charged scene,” NSW Ambulance Acting Superintendent Steve Vaughan told reporters on Monday.

“It took some time to ascertain exactly what had happened. There was a significant amount of scaffolding on the scene.”

Site worker Te Wihongi said he could hear the men yelling for help.

“We just tried our hardest to get all the scaffolding and bricks off them,” he told Seven News.

The fatal incident angered the construction union CFMEU, a spokesman said.

“Such tragedy should not occur. We will update members as more is known,” CFMEU construction and general secretary Darren Greenfield said in a Facebook post.

SafeWork NSW said it was unsure what caused the collapse and expected its investigation to take some time.

Construction company Ganellen said it was arranging support for workers on site at the time of the collapse.

“This is a tragic incident and our deepest sympathies go to the young man’s family, co-workers and friends,” a spokesman said in a statement.

“The welfare of our thousands of employees and subcontractors has always been a priority for Ganellen so this is devastating news.”

NSW Police are also investigating the man’s death.

Eight ambulances, plus police and fire and rescue crews, and the Care Flight helicopter were at the scene.