Gym users watched in horror as a woman collapsed suddenly in the middle of a group circuit-training class.
The woman, understood to be a 42-year-old mother from the North Shore, collapsed at the Albany Club Physical Gym about 9.30am on Monday.
The class trainer used a defibrillator to try to revive her, and two other women trained as nurses also tried to help.
The woman was taken to hospital but her life support was turned off hours later.
It is understood she suffered a brain aneurism.
Club Physical chief executive Paul Richards, who was at the gym, said: "The team and I were pretty much in tears after; the lady was known to most.
"This has been a very traumatic thing to happen for her family and our team, and members who were there at the time."
The woman's death comes just over a week after Pacific Media Network chief executive Tom Etuata, 46, was found dead in his Greenlane home after a brain aneurism.
Mr Etuata, a father of one, also worked as a fitness instructor at aLes Mills gym and lived a healthy lifestyle.