Friends, family and members of the public are today scouring the river on foot and by boat as they desperately try to find missing Calvin Harimate who has not been seen since falling off the back of a jet ski near Port Waikato last night.

Niece Alessea Herrick said her uncle had been jet skiing on the Waikato River just down from the Tuakau Bridge with about five friends about 8pm on Saturday.

The father-of-one was riding on the back of the jet ski when the driver did a sharp turn and he fell into the water.

By the time the driver turned around to collect Harimate he had disappeared and night was falling, she said.

Police found Harimate's life jacket, but they are still looking for the 46-year-old.

The devastated family had posted messages on social media asking for people to help in the search.

Along with more than a dozen family members on jet skis, quad bikes and on foot searching the river bank, several members of the public were also assisting with one person taking out a barge, a volunteer from canine search and rescue and a couple on a boat earlier today.

"We've pretty much been searching since 11.30pm last night. We had a break between 4am and 9am because of the fog."

At this stage his family planned to keep searching for him until he was found.

Herrick said Harimate was an experienced jet skier and swimmer. It was understood that he had been wearing a life jacket but it must have fallen off when he hit the water.

The search was focused on the area of the river between the Onewhero / Tuakau Bridge Road and the mouth of Port Waikato.

A police media spokesperson said a coastguard search was conducted this morning and nothing was located. LandSAR and a police RIB (rigid inflatable boat) are also involved at the scene.

Police would like to hear from anyone who saw a man on a jet ski last night in the river near Frost Rd.

Anyone with information or possible sightings is asked to contact the Police Maritime Unit on 0800 10 28 35.