Police said they had launched an investigation into Folole Muliaga's death, which happened within two hours of state-owned company Mercury Energy cutting power to her house Tuesday.
Mercury Energy's general manager, James Moulder, said the company was devastated by the woman's death and was conducting its own investigation to determine what happened. He refused to say how much money Muliaga had owed the company.
Muliaga, a schoolteacher with four children between the ages of 5 and 20, had been off work since February with an illness.
A Mercury Energy representative arrived on Tuesday at her home in the northern city of Auckland to disconnect the electricity, said Brenden Sheehan, Muliaga's nephew-in-law.
Sheehan said both Muliaga and her son told the technician she was dependent on the oxygen machine to stay alive and invited him into the house to see it. "Then he cut the power off," Sheehan told The Associated Press.
Muliaga began having difficulty breathing, became faint and then collapsed, he said. Paramedics were unable to revive her, and she was pronounced dead within two hours of the power being cut.
Moulder expressed his "deep condolences" to the family, and said the company was checking reports that it had been warned Muliaga needed power for the oxygen machine. The company restored electricity to the house on Wednesday after learning of her death.
Sheehan said the family's bills would prove Muliaga was trying to pay the account, and received no warning the power would be shut off. He declined to say how much she owed.
State Owned Enterprises Minister Trevor Mallard said there were reports the family had been warned about the overdue account.
"The correct authority to investigate this and sort out the facts is the police," Mallard said, adding the government would expect "full accountability" if the company was found to be culpable.
By Ray Lilley