A swimmer has died during an open water swimming event in Auckland, after suffering a suspected medical event.

The person was participating in the Kohi Summer Swim Series, which takes place at Kohimarama Beach every Thursday evening.

Event manager Haydn Woolley said first aid was administered by Lifeguards and St John medics immediately.

"Unfortunately their efforts were not successful."

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Woolley said his thoughts are with the family of the swimmer.

The swimming series, which has been running for nine seasons, is run over 16 Thursday nights every summer.

Competitors could pick how far they swam. Distances ranged from 250m up to 2.5km.