The wing, who played 63 Test matches for the All Blacks, had suffered from health problems since his retirement from the game in 2002 due to a rare kidney disease.
All Blacks team doctor John Mayhew confirmed that Lomu died at his home in Auckland from a heart attack.
“The final mechanism was something caused the heart to go into cardiac arrest, most probably a cardiac or pulmonary event,” said Mayhew in the Sydney Herald.
“Cardiac arrest is the final pathway of the heart shutting down. That’s all there is really.”
He exploded onto the global stage at the 1995 World Cup at the age of 19 and is best remembered for scoring seven tries in four matches in that tournament.
He retired from the game in 2007.
Lomu is survived by his wife Nadene and two sons.