The death of a competitor in a harbour swim race in Auckland yesterday has prompted concern at Water Safety New Zealand.

The man, who was thought to be in his fifties, was taken to the shore by a rescue boat but could not be revived.

Water Safety spokesman Matt Claridge says ocean swims have become very popular fitness challenges, especially in the 35-plus age group.

He says it is important people prepare themselves well before committing to an event.

Mr Claridge says swimming in the sea is not like swimming in a pool, and taking part in a race adds another level of stress.

A post mortem examination will be carried out on the man's body today or tomorrow.