The man, in his 40s, was pinned between scaffolding, propping steel and the concrete wall, which fell where he was working on a ladder, stripping boxing off a new concrete beam, about 11am yesterday.
Fire Service and St John staff worked for nearly an hour to free the man from under the slab.
A crane was called in to lift the concrete wall, which witnesses estimated weighed at least a tonne.
A St John spokeswoman said the man was conscious throughout and arrived at Dunedin Hospital at 12.15pm with serious chest injuries.
Amalgamated Builders manager of health and safety David Baker said the man had suffered several broken bones and other injuries.
"It was pretty fortunate considering [a concrete wall fell on him] ... but I guess he wouldn't say that."
Amalgamated Builders is building a new acute mental-health ward at Wakari Hospital.
The man was working in the area of a former radiotherapy bunker from which tonnes of concrete are to be removed.
The concrete had been cut into large blocks, ready for removal by crane once the roof was removed.
It was still unclear yesterday why the concrete wall had fallen, Mr Baker said.
The building site was closed for the rest of yesterday.
The Department of Labour had launched an investigation, a spokesman said.
