A Surf Lifesaving New Zealand spokeswoman said Waihī's emergency callout squad were searching in two IRBs.
Two Coastguard vessels were out searching, along with the Auckland Westpac Rescue Helicopter, which had carried out grid searches in the area, she said.
The search would continue for another two hours at least, the spokeswoman said about 5pm.
She said there was no information to hand about the age or gender of the missing swimmer.
Waihī residents have told the Bay of Plenty Times the missing person is a teenage youth who was visiting the area.
They understand the teenager lived in the Hamilton area and may have been with a school group when he went missing.
Another resident said understood the 17-year-old was last seen struggling to keep afloat near a rocky point after apparently becoming caught in a rip.
"Unfortunately, by the time the alarm was raised and rescuers returned to the beach, he had gone under," the resident said.
"It's very unfortunate. It doesn't sound good," he said.
It was one of a series of tragic incidents in waters around New Zealand on Friday.
In Napier this afternoon, a young boy drowned after being snatched by a wave as he played in the surf at Te Awa.
The Police Dive Squad located a body from the Whanganui River today. It is believed to be that of a swimmer reported missing on Thursday evening.
One person died today in Northland following an incident at Langs Beach. The previous day, a woman drowned at another Northland beach, Baylys beach.
