A teenage boy is feared dead after a boating accident in Victoria.

Two friends – both aged 16 – were fishing at Lake Mulwala, Yarrawonga, close to the NSW border, when a wave swept them from their tinny and dragged them underwater just before 9am.

It's understood the boys were visiting from elsewhere in Victoria.

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Two bystanders in their 30s and 50s who witnessed the incident managed to jump in and swim back to shore with one of the boys.

But when they returned to the water for the second boy they couldn't find him.

Search and Rescue officers are searching the lake with the assistance of Airwing, SES, and police divers, and will return at first light.