A worker at the Royal Adelaide Hospital site is in a critical condition after an accident this morning.
The construction union said the 54-year-old worker suffered a head injury and was found on a scissor lift just after 7:00am (CDST).
It was confirmed the man started his shift at 6.00am, but it was unclear how long he had been injured before being found by co-workers.
All work was suspended at the site as Safework SA carried out an investigation.
Union secretary Aaron Cartledge said the mood had been sombre and workers would meet again on Friday to discuss when they would return to work.
"A lot of guys knew this worker, he's been in the industry some time, is very popular on the site," Mr Cartledge said.
"This is a small industry - the South Australian building industry - there's a lot of guys that are devastated."
Earlier this morning Ambulance Services SA said the man went into cardiac arrest while on a scissor lift and fell.
He was resuscitated while being taken to hospital.