Police said the Auckland Westpac Rescue Helicopter and a helicopter from Tauranga were assisting with the search.
They are joined by three Coastguard vessels and a Coastguard plane, Surf Life Saving boats, and the naval patrol vessel HMNZS Taupo.
The search is focused on the open ocean.
Surf Life Saving teams are helping with shoreline searches.
The Police Eagle helicopter, which was used in the search overnight, has returned to Auckland.
Police said the water has flattened out this morning after a night of swells.
Waikato police Senior Sergeant Mike Henwood told Stuff that the group's dinghy had been found but the missing man was not with it.
He said the group, all from Rotorua, set off from Bowentown Harbour on Monday late afternoon but hit a wave at the bar and overturned about 8pm, Stuff reported.
He said only one life jacket was on board. The men put it on the boy and he, along with the 28-year-old, floated to shore just before midnight. Both had mild hypothermia.
A fisherman on the rocks spotted the two just offshore and phoned police.
"[The third man] clung to the dinghy and was washed further out to sea," Henwood told Stuff.
"The dinghy has been located, but no sign of that third person."
Bowentown Beach Holiday Park manager Nikki Pittams said the search was in the open sea just off Bowentown Beach, not inside the harbour.
"I was woken up at 3.30am by a chopper going over," she said.
"There is still activity over at the Coastguard, there've been cars and people there all night and all morning."
