The shark mauled a 49-year-old woman near a buoy which was marking the swimming area about 100 metres off one of the city's most popular swimming beaches.
A paddle boarder, who heard her screams, came to her rescue, and took her to shore.
With injuries to her hands, thighs and back, she was taken to hospital in a serious condition.
Sharks in New Caledonia's shallow coastal waters are usually more numerous in the summer.
It has been suggested that their presence has also increased because of the run-off following recent heavy rain.