A fault with Meridian Energy's billing scheme has left 9000 of its customers who thought they were in credit actually owing money.
Under the Level Pay system, users pay the same amount on a regular basis all year round to iron out the seasonal peaks and troughs in power bills.
The fault means money has not been taken from customers' accounts, which means automatic payments have not been deducted from customers' bills, leaving people owing large sums.
Meridian says the problems started last September but the earthquake slowed its progress on fixing the system's faults.
It says 814 customers had a debit balance of more than $500.
Meridian's general manager of retail, Bill Highet, says the system will be fully operational by October.