Two children clung on to each other for dear life after a terrifying ordeal with two massive orca whales off Waiheke Island this afternoon.

A video sent to 1 NEWS by viewer Shane Watt shows the two children swimming frantically as the killer whales headed straight for them, before swimming past either side of them and out to sea.

Mr Watt says he was at a house looking down as the scene unfolded before him at Enclosure Bay, on the northern side of the island.

"The kids were obviously very, very frightened," he said.

"We were terrified just watching. It took us a minute to calm down afterwards so I can't imagine what the adrenaline would've been like for them."

Two kayakers went out to retrieve the two children, Mr Watt said.

The close encounter happened at around 2.30pm.

Yesterday, a dead whale caused a feeding frenzy for sharks off the coast of Waiheke Island.

It's not yet know what killed the whale.