Police say a young boy had been fishing with his father and brothers before he was swept into the ocean off the NSW Central Coast.
The alarm was raised about 5:15pm on Sunday after the 11-year-old was spotted in rough conditions off The Entrance beach.
Inspector David Piddington, from the Tuggerah Lakes Police District, said the 43-year-old father tried to cross The Entrance Channel from the north side with his four boys aged 11, nine, seven and three.
There are signs warning the public about the dangers of trying to cross The Entrance Channel. (ABC Newcastle: Shauna Foley)
The ABC understands the father was carrying one or two of the younger children as the family tried to wade across.
A strong current swept the eldest boy out into the ocean.
Two bystanders looked after the three younger children, while the father tried in vain to reach his son.
Rescue helicopters, water police and surf life savers helped search for the boy until after dark, with efforts resuming this morning.
Surf life savers were called in to help search for a missing boy at The Entrance. (Supplied)
Inspector Piddington said the search would extend as far south as Avoca Beach, about 20km away, and involve helicopters, boats, jetskis, police divers and surf life-saving resources.
The family, who are from a non-English speaking background, were visiting the area from Sydney.
"The family are very distraught," he said.
"They are being well supported by our family liaison officer, chaplains and we'll assist them as much as we can moving forward."
Inspector Piddington said the family had been fishing before the incident occurred. (ABC Central Coast: Shauna Foley)
Inspector Piddington said some of the extended family had travelled from Sydney to offer support and help translate.
Central Coast Surf Life Saving chief executive officer Michael Hyslop said the incident happened just minutes after patrols finished for the day.
"The beach is patrolled. Unfortunately it's flags down at 5 o'clock, so this incident occurred outside of our patrol times," he said.
Mr Hyslop said all resources were being engaged to find the boy. (ABC Central Coast: Shauna Foley)
He said The Entrance Channel was a "dangerous" area.
"The channel is definitely a hotspot for us, something we patrol with more intensity than other areas," he said.
Mr Hyslop said there were signs in place warning beachgoers about the dangers posed by the channel.
Two fisherman drowned in the same area several years ago after falling overboard.