Emergency services were called to the Lake Macquarie Yacht Club at Ada Street, Belmont, about 4.45pm on Saturday, responding to reports that a child had fallen into the lake while sailing.

The boy appears to have been training on the water in a "Super-Eleven" yacht when it capsized.

The two-person vessel then appears to have caught the boy in some ropes before righting itself, say police. The child was wearing a life jacket during the incident.

According to Alan Patterson, general manager of the Macquarie Yacht Club, the incident was seen by a local fisherman who alerted a nearby yacht from the Yacht Club. The crew administered CPR on the boat and brought the boys back to the clubhouse for medical treatment.

The boys' yacht did not leave from the Lake Macquarie Yacht Club, and was not involved in any official regattas, according to Mr Patterson.

A clearly emotional Mr Patterson said the club members were in shock.

"This morning I'll go check on the crew [of the vessel that picked up the boys] and see they are all right," he said, adding that "the crew did a sterling job".

He said the club will "make sure that everyone that needs counselling receives it."

Ambulance paramedics gave the boy first aid before taking him to John Hunter Hospital, where he remains in a critical condition.

Police from Lake Macquarie Local Area Command are investigating the incident and speaking with witnesses at the scene, including the second boy on the boat.