Emergency services rushed to the scene, near Glenavy north of Oamaru, around 8.30 after reports of someone in trouble in the water.
Around 9.45am police said a person had been rescued. It was later confirmed that the person had been pulled from the water by a helicopter crew.
A St John spokesperson said the person had been taken to Dunedin Hospital in a critical condition.
The incident comes after a whitebaiter had a lucky escape in the same area when he was swept out to sea about three weeks ago.
The man was wearing a lifejacket, and made it back to shore by himself.