A heart specialist tried desperately to resuscitate a fellow competitor who died from a suspected heart attack during the popular 2.8km Auckland Harbour crossing swim yesterday.

The drama unfolded after a 36-year-old Auckland man ran into difficulty about 50 minutes into the race. A Mairangi Bay surf lifesaver, patrolling the race on a jetski, flagged down a safety boat after noticing the man, who was only two minutes' boat ride from the finishing post at the Auckland Viaduct.

The Auckland cardiologist tried to resuscitate the man on board the boat. It is believed he suffered a heart attack while in the water.

Regional Communication Centre team leader Murray Bannister said St John's ambulance staff then performed CPR on the man for 30 minutes before he was taken by ambulance to Auckland Hospital where he later died.

His name had not been released at edition time.

Race organiser Scott Rice said all safety and medical procedures had been followed. The man had received immediate medical treatment as soon as the race's surf lifesaving escorts realised something was wrong. Patrolling the water were 34 Mairangi Bay surf lifesavers, safety staff on 16 Thundercat boats, police and the harbourmaster.

Mr Rice said he was saddened by the man's death and his thoughts were with his family and friends.

The harbour crossing started from Stanley Bay Beach on the North Shore, and attracted 900 entries - 400 more than last year.

Devonport competitor Belinda Carpenter, who completed the race, hoped the man's death would not affect future events. "It was the safest race by far I have competed in," she said.

"The sea was a little choppy but because there were so many rescue people around, you never felt alone."

About 22 competitors failed to finish the race, won by Olympic swimmer Moss Burmester, and were picked up by the safety boats, Mr Rice said.