A Queensland Ambulance spokeswoman said the man was taken to Princess Alexandra Hospital after the fall about 1.15pm on Tuesday at Dutton Park.
A Metro South Health spokeswoman said,as of shortly before 5pm,the man remained in a critical condition at the PA.
The man was injured at the Cross River Rail site at Boggo Road,off Joe Baker Street.
Paramedics responded after reports he fell about eight metres.
The Cross River Rail Authority said it was aware of the incident and was investigating.
A spokesperson said the site had been shut down and would remain closed on Wednesday.