''I was fully expecting to find a dead body,'' he said.
The man declined to be named and played down his actions, saying anyone would have rushed to the woman's aid.
''I'd say it's typical of Dunedin.''
Another man who assisted the woman, who appeared to be in her 50s or 60s, said she had extensive injuries.
The car's engine bay had disintegrated in the crash, ejecting the battery and reducing the threat of fire, he said.
St John spokeswoman Chrissy Hamilton said the woman was taken to Dunedin Hospital in a serious condition.
A senior firefighter at the scene said speed was believed to be a factor in the seriousness of the crash.
Police were on scene interviewing several witnesses.
Both lanes of Kaikorai Valley Rd were closed for at least an hour.