Spark is contacting 135,000 ex-customers in an attempt to give them money back.
Spark launched the initiative to return account credit to former customers - ranging from a few cents to over $100 - amid discussions with the Commerce Commission about billing system problems.
The move will cover ex-customers from the past seven years and those affected will get an email or letter from the telco.
"There is a range of reasons why a former customer might have a credit left on their old account. Some customers were left in credit at the end of their account termination process due to the way their final bill was calculated," said Grant McBeath, acting chief executive of Spark Home Mobile and Business.
"Some customers didn't cancel an automatic payment before leaving or mistakenly paid their final bill twice. Or in some cases, a refund from a billing error on Spark's part was discovered after customers had left Spark," McBeath said.
"Whatever the reason, the credit remains on these old accounts and we want to give the money back. This money is held for the customer until it is paid back."
"We want anyone with a credit to get their money back. Once a customer has left us, it becomes harder to track them down and then verify their identity to make sure we give the credit to the right person – while also making it as easy as possible to claim their credit. So, we're launching a public campaign and contacting former customers who have a credit from the last seven years to remind them of this."