Barnaby Jack, a computer hacker who forced bank ATMs to discharge cash and sparked safety improvements in medical devices, has died in San Francisco.
Mr Jack, from Auckland, was found dead in his apartment. He was 35.
Computer experts in New Zealand say they are shocked at his death.
Mr Jack gained notoriety for hacking into ATMs and for showing that people's pacemakers could be interfered with from nine metres away.
But his friends say this was only done to demonstrate security concerns that manufacturers should put right.
A New Zealand computer security expert, Michael Forbes, says Mr Jack was a great person and his death is a tragic loss.
Another computer industry friend, Mark Piper, says Mr Jack was hugely respected in the industry.
His employer in California, Jennifer Steffens, says she wants to turn sadness at his death into a celebration of his life and work.
Mr Jack made his mark as a "white hat" hacker - finding holes in security before criminals do.
He became famous in 2010 after a demonstration of "Jackpotting" - getting ATMs to discharge money. A clip of his presentation was reportedly viewed more than 2.6 million times on YouTube.
Two years ago, he turned his attention to medical devices.
He was due to appear at a hacking convention in Las Vegas next week.