The 11-year-old was walking with his three siblings and father at The Entrance on the NSW Central Coast when disaster struck about 5pm on Sunday.
The family were visiting from Sydney and fishing in the area when it's believed they took a shortcut across a channel between an estuary and the ocean before the boy was swept away while the tide was going out.
"Lots of people try and move across that channel, it is not advised though," Inspector David Piddington told reporters on Monday.
While two bystanders looked after the other three children, the father tried to reach the boy but was unsuccessful, police said.
An extensive land, air and sea search for the boy resumed about 8am, involving local police, surf lifesavers, water police and rescue helicopters.
The search area extends about 15km south to Avoca.
Marine Rescue NSW inspector Steve Raymond said volunteers in two vessels resumed conducting parallel line searches under direction from police amid lumpy offshore conditions on Monday.
Attempts to find the boy on Sunday night were called off when it got dark.
Insp Piddington said the boy's family was very distraught.
Police liaison officers and extended family members were assisting with translation and support.
"We'll assist them as much as we can moving forward," Insp Piddington said.
The Entrance is at the mouth of Tuggerah Lake and is a popular weekend and holiday destination about 100km north of Sydney.
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