Genesis Energy says the Queen's Birthday holiday was not programmed into its software, compounding a systems failure which left hundreds of people without power during the coldest weekend of the year so far.
The systems failure affected customers in the lower North Island over the long weekend who use the pre-pay "in Charge" method.
The electronic payment system allows customers to top up their power. Some of the 10,000 customers on the plan were unable to buy electricity until 12pm on Monday.
Genesis public affairs manager Richard Gordon told Checkpoint on Tuesday that 186 people who use the pre-pay method are being compensated.
Mr Gordon says customers affected were without electricity for six hours and have been offered "goodwill credits" of up to $50. Technicians are working to make sure the problem does not happen again.
Genesis confirmed on Tuesday that 16 customers who required power for medical reasons were affected by the computer glitch. Mr Gordon says contractors were sent to their homes to restore their power supply.
Power companies were urged to be more cautious about vulnerable customers following the death two years ago of an Auckland woman who used an oxygen machine whose power was disconnected.
Mr Gordon says Genesis called all customers medically dependent on power in November last year and asked them to switch from the top-up system to a contract.
He says Genesis has had trouble with the pre-pay system in the past and a full review is being carried out.
Anna Stone, of Palmerston North, was one of the customers left without power. She says her family had no power for about five hours and she phoned Genesis several times about the problem.
Energy Minister Gerry Brownlee says he is appalled that electricity was cut to some of the publicly owned company's customers.
He says Genesis has massively let down the people who were making an honest attempt to pay for their power.
Mr Brownlee says the company will need to work hard to restore the integrity of the pre-pay scheme if it expects people to sign up to it.
[audio:http://www.radionz.co.nz/audio/national/ckpt/2009/06/02/genesis_customers_to_be_compensated_up_to_$50]Listen to item on Checkpoint