The mother of a Rotorua man missing on Lake Rotorua says she suspects her son had an epileptic seizure while trying to swim for shore.

William Rouppe van der Voort, 24, went fishing with his flatmates around 5pm last night, his mother Julie van der Voort said.

The lake was "like glass" so none of the men had lifejackets. Her son was a strong swimmer and all the men were experienced on the lake, she said.

She said she understood the lake began to get choppy so the men decided to head into shore. However about 500m from land, their aluminium dinghy capsized.

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Two men stayed with the boat while three, including her son, started to swim for shore.

Mrs van der Voort said she believed her son, an epileptic, must have suffered a seizure while swimming.

It all happened within about 20 minutes, she said.

Mrs van der Voort said she spoke to the rescued men late last night about what happened, after they left hospital.

Coastguard resumed the search for Mr van der Voort at first light this morning and the Police National Dive Squad is expected to join the search later today.