Spark says some customers who left the company in the past seven years still have credit left on their old account - and it wants to return it.
The company is launching a campaign to contact former customers. Over the next week, around 135,000 people will be sent an email or letter prompting them to apply for a refund. These credits range from a few cents to over $100.
Acting CEO of Spark Home Mobile and Business, Grant McBeath, says there is a range of reasons why a former customer might have 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," he says.
"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.
"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."
Spark says it launched the campaign after "discussions" with the Commerce Commission.
"While this investigation is ongoing, it prompted Spark to look take a look at the broader issue of account credits," Mr McBeath says.
"As a result, we decided to take a more public, proactive approach and run a public campaign to encourage former customers to claim their credits."
Customers with credit can choose to have their money sent to a personal bank account, transferred to a different Spark account, or donated to one of the worthy causes at the Spark Foundation.