Gauteng Emergency Services' Lesiba Malotana says construction work was being done on the roof.
It is unclear at this stage what caused the structure to collapse, although bystanders on the scene have confirmed that the roof above the entrance was now under construction with new roofing being installed.
Roughly 30 emergency personnel searched through rubble to find people injured.
Videos posted on social media show chaotic scenes inside the hospital ward.
State hospitals in South Africa have been struggling with persistent underfunding for staff, equipment and facilities.
Rescue workers confirmed that the number of injured people is five, but rescue workers are continuing to dig through the rubble to look for anybody who might be trapped.
The rest hospital is safe, she said. They were sealing the roof when the roof collapsed.
She said rescue workers were busy working on site to ensure no one was trapped underneath the rubble.
Gauteng provincial health minister Gwen Ramokgopa said workers were on the roof when it collapsed into a public area of the Charlotte Maxeke hospital in central Johannesburg.
Structural defects, including persistent ceiling leaks because of faulty heating pipes, have long plagued the hospital with reports surfacing as far back as 2012 that the hospital was at risk of collapse.
"The collapse took place in the passage near maternity department on the hospital street".
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