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Medically dependent customers were "strongly advised" not to choose pre-pay, because of their need for a continuous power supply, Contact strategic communications and partnerships general manager Nick Robinson said.
Contact had 365 pre-pay customers in Dunedin. It does not supply other parts of Otago with pre-paid power.
If customers raised an alert, company staff exercised discretion, and power to those who were medically dependent would not be disconnected.
They also exercised discretion for other pre-pay clients, depending on the circumstances.
If medically dependent customers did not raise the alarm, the power would go off, because "that's the nature of how pre-pay works", Mr Robinson said.
However, the company went to "great lengths" to ensureit had up-to-date information regarding its medically dependent customers, he said.
A University of Otago survey of pre-pay power users in 2010, which included Dunedin and Invercargill consumers, showed 53% experienced "self-disconnection" (running out of money) in the previous year. Followed up last year, 45% of respondents reported self-disconnection in the previous year.
Highlighted in The New Zealand Herald yesterday, the survey had not looked specifically at the issue of medically dependent power consumers.
New Zealand's lack of electricity industry regulation meant those with a medical condition were vulnerable on pre-pay power, Prof Howden-Chapman said.
They were potentially the "tip of the iceberg", in terms of vulnerable power users, which included the elderly.
Dunedin Hospital respiratory consultant specialist Prof Robin Taylor said while it was inadvisable for medically dependent customers to choose pre-paid power, it was ultimately a personal choice.
It was vital the power companies warned people of the risks, he said.
Anglican Family Care Centre director Nicola Taylor said those on pre-pay power "know where they stand", and could budget more effectively.
The centre administers Dunedin's Consumer Electricity Fund, which this financial year had allocated about $20,000 of its $170,000 budget.
She did not know how much of that, if any, was used to assist those on pre-paid power.
Methodist Mission chief executive Laura Black, of Dunedin, said the real issue was that many people did not have enough money to pay escalating power bills, regardless of payment method.