Spark has already refunded almost 20,000 former customers who had credit on their accounts.
Last month, the telco launched an 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.
Of the almost 135,000 former customers contacted by mail or email at their last known address, around 15 per cent have so far claimed their money.
Grant McBeath, the acting CEO of Spark Home, Mobile and Business said this was a good start toward making sure customers get what's owed to them.
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"We're keeping the claims website open and we hope many more visit it and give us the details we need to confirm if they're owed a refund," McBeath said.
McBeath said there were a range of reasons why a former customer might have a credit left on their old account.
Some were left in credit at the end of their account termination process due to the way their final bill was calculated. Others didn't cancel an automatic payment before leaving or mistakenly paid their final bill twice. In some cases, a refund from a billing error on Spark's part was discovered after customers had left.
Customers are given the choice to have the money sent to a personal bank account, transferred to a different Spark account or donated to charity.