The 17-year-old, identified in media reports as Tyler Whitton, died after falling several metres onto concrete at the CS Development Group building site in the inner-city suburb of West End on Tuesday.
The teen was rushed to hospital with serious head injuries and died on Thursday surrounded by friends and family.
CFMEU state secretary Michael Ravbar said the death could have been prevented.
"I've been around for a number of years, but this is probably the worst fatality I've ever seen," he told reporters.
The incident comes after the death of a surveyor at a Seymour Whyte site at Victoria Point, east of Brisbane.
"We've had a shocking week in Queensland in regards to fatalities in this state," Mr Ravbar told reporters.
He said the union had raised concerns over safety issues at the West End site.
"There was no head protection, there was no scaffolding," he said in a statement.
"That site has been stopped on numerous occasions."
Workplace Health and Safety Queensland said construction had stopped and investigations were under way.
"WHSQ understands all works on site ceased immediately after the incident except a concrete pouring operation in a different area of the site," the department said in a statement.
Compliance notices had been issued requiring the rectification of a number of high-risk issues before work could restart, it said.
CS Development Group said counselling was being provided to workers and the company was co-operating with workplace safety authorities.
"CSDG expresses our deepest sympathy and concern for the family of a worker for an external contractor involved in a tragic accident at our West End site," it said in a statement.
"CSDG is committed to the safety and wellbeing of workers, contractors and the public, and employs two full-time safety staff."
Mr Ravbar said serious incidents were happening regularly on building and construction sites across the state, and he renewed calls for an overhaul of the workplace safety regulator.
"It's unfortunate that you have to have these tragic events to get the government's attention," Mr Ravbar said.
"You've got a government, you've got a regulator, they're accountable, they're responsible, they've got a job to do for the community of workers in this state, and they have failed us miserably."
In July, a 54-year-old worker was critically injured in a fall at the Cross River Rail construction site in Dutton Park.
That incident prompted protests from the union, which claimed safety standards were "grossly inadequate"Â at the multibillion-dollar project, while Transport Minister Mark Bailey promised a "safety reset" at its worksites.
Two weeks ago, hundreds of building and construction workers demonstrated outside parliament house demanding mass changes to "outdated" safety regulations.
The office of Industrial Relations Minister Grace Grace said she would meet CFMEU officials next week.
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