The 17-year-old has not been seen since he ended up in the water yesterday, and was hit by another machine.
Detective Senior Sergeant John Wilson says the police dive squad is at the lake and ready to commence searching.
"At the moment we have kayaks out doing a shoreline search. Coastguard is on standby to assist with the dive squad."
Ten volunteers on kayaks began searching early this morning. It is understood the missing boy is a local high school student.
Mr Wilson says the lake will be closed to members of the public until further notice.
Maritime safety inspectors have called for jetski and boat users to take greater care following yesterday's accident and another, near Tairua, on Friday evening.
They say personal responsibility from all boat and recreational drivers is paramount.
"The key factors for safe boating are not rocket science," Maritime New Zealand safety inspector Jim Lilley said.
"Just as there are rules for the way we operate motor vehicles on our roads, there are rules that follow us out on the water and govern how we operate our vessels, with awareness of others and care for yourself, your passengers and those in the water near you at the top of the list."
Mr Lilley said boat and jetski users needed to slow to 5 knots, or about 9km/h, either when they came close to other vessels or swimmers, within 200m of the shore or a dive flag, or if they had passengers bow riding.
Every skipper was required to maintain a proper lookout and to maintain a safe speed at all times.
"These are the main things anybody out on the water should remember, and everyone should enjoy the rest of their summer holidays and we can avoid any more accidents," he said.
Rotorua Detective Sergeant John Wilson said the Rotorua teen missing at Lake Okareka did not appear to be wearing a life jacket when the jetskis collided.
The Herald on Sunday was told the boy fell from one jetski and was hit by a following one.
Rotorua harbourmaster Ross Powell said he was saddened searchers were dealing with a "needless" tragedy.
"We have been drumming home the need for water safety and the importance of wearing life jackets but the message doesn't seem to be getting through," said Powell.
Wellington's Dave Corbett took his own runabout to help search. Wiping tears from his eyes, he said: "All I could think about was what if it was one of my kids. I don't think they are going to find him alive. It's been too long. I just feel for the family now.
"What a horrid memory of their holiday when they should be going home with great memories of this beautiful spot."
Lake ranger Mike Goodwin said: "Someone has come off a jetski and another has hit him hard at a big speed. He would have had very serious head injuries. There have been reports from some people who were at the lake at the time that they were going very fast and being irresponsible," he said.
"Jetskis are a lot of fun for people. They're like trail bikes on water. They are a lot of fun but very dangerous and very powerful. Some are capable of up to 100km/h," he said.
"I don't think they understood the rules. They shouldn't have been travelling at speed so close together. The rule is five knots within 50m of another vessel and personal flotation devices must be worn."
Lake Okareka Lodge manager Diana Moore confirmed the two jetskis were travelling close to each other when the collision happened.
"A passenger has come off and another jetski ran into him and he disappeared under the water. He was not wearing a lifejacket."
A resident, who declined to be named, said she had seen a group of young men riding recklessly in a speed restricted area. "They went hooning past me and I yelled at them to slow down but of course they didn't."
- HERALD ON SUNDAY, NZPA, NEWSTALK ZB