The Westpac Rescue helicopter and Oxley police were called just after midday to the northern end of Lake Keepit Dam near the Manilla Ski Gardens after reports a ski-boat had hit a tree.

The driver of the boat, a 68-year-old Tamworth man, has been flown to Tamworth Base Hospital in a serious condition, after he was ejected through the windscreen of the boat.

Three other adults were taken by ambulance to Tamworth Base Hospital.

Two women in their 50s received minor injuries and lacerations, along a man, also in his 50s who suffered neck injuries.

Long-weekend holiday makers had begun to arrive at the popular camping spot, near Shaley Point when the incident occurred.

Witnesses reported seeing the boat come to a violent stop on the lake and the passengers flung forward.

A partially submerged tree was believed to be the cause of the accident, with the lake's levels at just 42%.

Tamworth paramedic Ray Tait, who attended the scene, said the three passengers were lucky to escape with only minor injuries.

"It was a huge impact, they were very lucky to escape further injury."

Oxley police are investigating the accident.