"The young boy released her, he said go, he pushed her away. Then she was drifting in the water as well."
Lutau, who heard the boy cry out for help while she was swimming but couldn't leave her children, said those on the beach were shouting advice to the woman as she struggled.
"We said, float, float, lie on your back and she managed to swim in."
Lutau said the boy drifted further and further out. "And then he just went under until we couldn't see him any more."
Family called emergency services to the remote West Coast beach around 1.30pm.
The Huia fire brigade was first to arrive, followed by lifeguards from Karekare - the nearest manned beach - with jet skis and inflatable rescue boats.
Whatipu, a black sand beach on the north side of the Manukau Heads known for its strong currents, does not have its own surf lifesavers. It is an hour's drive from Auckland down a windy, unsealed road.
Two Coastguard vessels, the Westpac Rescue Helicopter, and several local boats joined the search, largely focusing on the area between Ninepin Rock and Paratutae Island where the boy was swimming.
The woman who tried to save the teen was taken back to Auckland by St John Ambulance. Another woman was also treated by paramedics after she collapsed.
The rest of his family waited, distraught, at the water's edge.
One man, thought to be his brother, refused to leave even after most of the rescuers had gone.
Police said one person was kept in hospital overnight for observation.
The search resumed this morning around 8.30am with Land Search and Rescue and the Police Eagle helicopter.
Coastguard Air Patrol left Ardmore to help around 11.15am.
A family member said they were having a barbecue at the campground nearby when they heard the boy was missing.
"We came down straight away when we heard he was swept out," he said.
As she was leaving the beach, Lutau said she would take her children home to talk to them about what had happened.
"They don't really understand yet," she said.
Police said they would focus on continuing to support the boy's family.
During the past seven days there have been three drownings - a man in Lake Taupo on Christmas Day, a free diver in Wellington on December 23, and a 20-year-old man who got into trouble while swimming at Maraetotara Falls in Hawke's Bay.