It was likely to be some time before the body was found, he said.
"Generally, in west coast conditions, it can be hard to find people.''
Police spokesman Kevin Loughlin said the boy was 15.
It is understood he had been swimming between the flags with his brother at Bethells Beach, but the pair left the patrolled area and went over the sand dunes to ``extremely dangerous'' O'Neill Bay.
A man at Waitakere Bay, the small bay between Bethells and O'Neill, said that just before 3pm, a teenage boy came "screaming down'' over the sand dune towards him.
"He was sobbing, totally hysterical ... He said, 'My brother's gone, my brother's dead'.''
A woman believed to be the victim's mother then went to O'Neill Bay but collapsed and was taken back to Bethells Beach, the witness said.
Another woman said the teenage boy was sobbing and appeared to slap his face in despair before onlookers took him to raise the alarm with the lifeguards at Bethells.
Lifeguards were on the scene within 10 minutes of being alerted. Rescue boats from Piha and Muriwai joined the team from Bethells in the water, and three aircraft _ including the Auckland Westpac rescue helicopter and the police force's Eagle _ combed a 2km stretch of coastline.
A paramedic on the Westpac chopper, Marcel Driessen, said they searched for an hour and a quarter before being stood down about 4.45pm.
Four inflatable rescue boats also joined the search.
As it continued, the woman believed to be the boy's mother and a teenage girl sat with a police officer on the beach near the spot where he was last seen.
Soon after, the group walked slowly away from the beach, the woman wiping tears from her eyes and the teenage girl holding a crying toddler in her arms.
Lifeguards and police drove them and another teenage boy back to Bethells Beach, where Victim Support staff had been called to assist.
Regional lifeguard supervisor Jason Harvey said few details could be given because many of the boy's family did not know of his disappearance last night.
Mr Cahalane said O'Neill Bay was extremely dangerous and not recommended for swimming. Until yesterday's incident, it had been a ``really quiet summer'' at Bethells.
By yesterday morning, 12 people had drowned this year, compared with 15 at the same time in 2011.
Water Safety New Zealand chief executive Matt Claridge said the boy would be one of the youngest victims this year if he had drowned.
"Generally January has the highest number of drownings of the year but it's still disappointing when you see a drowning come in.
"The further through January you get, the more incidents involving children occur and that's related to more parents being back at work.''
