It is understood that the four teenagers have later been found, apparently safe and well, near a bridge in the gorge.
A local helicopter pilot spotted the teens from above and liaised with police.
The area is believed to be difficult to access by road, and air access may be needed to bring them out of the area.
A police spokeswoman said the teenagers, apparently aboard kayaks, a lilo and an inflatable doughnut, had set out about 2.15pm and had been expected to reach the bridge, where family members were waiting, about ninety minutes later.
Police were notified about 5.55pm when they had not arrived as expected, and a search and rescue party had been put together and it was planned to put up a helicopter in a bid to locate the teenagers in the Manuherikia River Gorge.
