A 19 year old remains in a serious condition in hospital, after being swept out to sea off Auckland's notorious Piha Beach.

She and a 17 year old girl were swimming when they were caught in a strong rip yesterday afternoon, and dragged as far as 200 metres offshore.

Police said members of the public heard their screams, with two people racing to their aid - dragging the unconscious 19 year old to shore.

The 17 year old made it back to shore and was treated for shock, but was otherwise fine.

The Police Eagle Helicopter landed on the beach, with officers performing CPR until Westpac Rescue Helicopter staff arrived and took over.

After a considerable amount of time the young lady regained a pulse and was flown to Auckland Hospital in a serious condition.

Off-duty lifeguards from Piha Surf Lifesaving Club, St John Ambulance and locals also assisted at the scene.