The 16-year-old Christchurch youth had travelled to Dunedin with friends for the New Zealand surfing championships at St Clair.
Yesterday afternoon, Billy and two friends headed to Allans beach, 5km southeast of Portobello, for a surf, but the others opted out.
Emerging from the water after 90 minutes, he assumed his mates had got sick of waiting and headed back to St Clair, so he hitched a ride with other surfers.
However, his friends were sightseeing and returned to the beach about five minutes after he left.
A helicopter and search and rescue craft covered an area of more than 100km while a fixed-wing plane was on stand-by and a container ship was diverted from its course by Maritime New Zealand.
Friend Daniel Wakefield described the experience as "gut wrenching" as he and Stephen Weir searched for their friend and waited for news from the rescue team.
As they were all experienced surfers, it was hard for them to understand how he had gotten into trouble.
"We could not figure out what happened to him. He knows what to do," they said.
Police tracked him down, safe and well, at St Clair's Esplanade about 8.40pm yesterday, after he was called to shore to take a call from police on a friend's cellphone.
Senior Constable Lox Kellas, of Portobello, said the call to emergency services was made with "good intent".
Finding Billy safe and well was a "positive result" for the search.
Billy thought it was "pretty amazing" a major search operation had been launched for him.
"People actually care about me," he joked.
Acting Senior Sergeant Ed Baker said no charges would be laid.
