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.