The death of an 18-year-old apprentice who was killed in a catastrophic accident at a Sydney construction site has been described as shocking and heartbreaking by Australia's political leaders.

Christopher Cassaniti was crushed to death on Monday after scaffolding, 15 to 17 metres in height, collapsed on him and a co-worker at a construction site in Macquarie Park.

“I am just so saddened about the death of young scaffolding apprentice Christopher Cassaniti. To his parents and workmates my heart goes out to you,” Prime Minister Scott Morrison tweeted on Tuesday morning.

Opposition leader Bill Shorten also sent his condolences and described Mr Cassaniti’s death as shocking.

“As a father of three, it is incomprehensible what his family must be going through at this time – and I think I speak for everyone when I say they are in our thoughts,” Mr Shorten tweeted on Tuesday.

Paramedics were initially unable to reach the formworker who was trapped under rubble and was pronounced dead at the scene a few hours after the incident.

His colleague, a 39-year-old man, suffered injuries to his lower body and was taken to Royal North Shore Hospital where he remains in a stable condition.

Days before the incident Mr Cassaniti’s mother sung the praises of her son, who turned 18 last Wednesday.

“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 had posted to Facebook.

On Monday evening, friends of the former St Patrick’s College student posted tributes in memoriam.

“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. You were full of positivity, always smiling, always seeing the best in everyone,” Bella Mineo said.

A spokesman for construction company Ganellen expressed sympathy to the family, colleagues and friends of the young man.

SafeWork NSW and police are now trying to work out why the scaffolding at the Lachlan’s Line apartment complex collapsed.

Construction union CFMEU national secretary Dave Noonan said such an incident shouldn’t happen if scaffolding is erected, maintained and loaded property.

“Tragedies like this just shouldn’t happen and we can’t imagine the grief and sorrow that family are in at the moment,” Mr Noonan told Seven’s Sunrise on Tuesday.