Two women were saved from drowning when off-duty lifeguards sprang into action at Piha.

Rescuer Jackson Smith didn't hesitate when he heard two girls were in trouble in Piha's surf on Tuesday afternoon.

He stripped down to his shorts and ran into the surf to save them.

The 18-year-old Red Beach lifeguard from Greenhithe had been walking along North Piha beach with his girlfriend when they come across an hysterical girl.

"Me and a few others ran towards her and asked what was wrong," Smith says.

"She was in shock and crying about her friend and her sister who were lost in the waves. She was talking about how the waves were coming too fast," he says.

A king low-tide and large rolling waves made conditions tricky, with a strong rip sweeping the girls 100-200 metres offshore.

Smith and others ran along the beach until they saw a girl in the water. He went in to find the 17-year-old waist deep.

"She was in shock, but she was walking, and another man helped her out."

The girl told them her 19-year-old friend was face down in the water and "may have drowned".

Smith couldn't see the other girl but noticed a strong rip running north and figured the girl would be 100m down the beach and further out to sea.

"I swam out and found her face down."

He lifted her up and was then joined by an American tourist, the two dragging her to shore.

"She was completely unconscious, but breathing weakly.

"With every breath out, water was pouring from her mouth and nostrils."

Smith was shocked when he saw the girl's condition.

"I just realised how real the situation was. It was really scary, but I knew I had to do something."

Three years training at the Red Beach Surf Life Saving Club meant his training kicked in, and rescuing the girl and tending to her came naturally.

Smith and the American did resuscitation for around 10 minutes, then another off-duty lifeguard assisted them.

The police Eagle helicopter then landed and its crew defibrillated the girl before the Auckland Westpac rescue helicopter arrived.

After some considerable time the young woman regained a pulse and was flown to Auckland Hospital in a serious condition.

Her 17-year-old friend was treated for shock.

Other off-duty lifeguards from the Piha Surf Life Saving Club, St John Ambulance and other members of the Piha community also assisted.

"Sadly, the young lady remains in a serious condition in Auckland Hospital and our thoughts are with her and her family," Waitemata Police District Commander Bill Searle says.

Searle praises the actions of rescuers and others involved in the girl's revival. "We now dearly hope she pulls through and makes a full recovery."

Police want to congratulate Smith, an Auckland University student studying law, but didn't know who the rescuer was until now.