Spark today pleaded guilty to over-billing charges brought under the Fair Trading Act.
The charges were filed by the Commerce Commission over two issues that affected more than 100,000 customers.
Charges related to a third issue were dropped.
Penalties will be set at a later hearing. Spark faces fines of up to $600,000 per charge, or $1.2 million in total.
The ComCom filed charges in July, saying:
• Letters offering new Spark customers a $100 account credit for subscribing to a particular broadband plan failed to mention the offer could only be redeemed by phoning Spark. The offers allegedly created the impression that customers signing up online would receive the credit, when they would not, the ComCom claims;
• The telco over-billed ex-customers. From June 2, 2014, Spark's terms and conditions said charges would stop 30 days after a customer gave notice to terminate their contract. However, a customer's final bill included charges for the entire next monthly billing period regardless of when the Spark service stopped.
The third charge, which was ultimately dropped, related to customers being overcharged for broadband data in 2015 when a fault in Spark's broadband network misrecorded customer data usage.
Spark said earlier that around 135,000 customers were affected by the overbilling, with amounts varying between $1 and $100. It had proactively approached customers and encouraged them to claim a refund (although a public campaign did not begin until after a nudge from the ComCom).
When Spark updated in July, a total of 20,000 customers affected by the finally billing issue had claimed refunds totalling $1.1m (the company declined to provide any update to that figure today).
The telco said had repaid a total of $216,937 to all 5325 customers affected by the hardware failure issue and a total $46,300 to the 463 affected by the "welcome letter issue."
"These were all system-based errors caused by genuine mistakes, with no malicious intent involved on the part of Spark," Spark managing director Simon Moutter said.
The company has since overhauled its tech systems as part of its "agile" restructure, the Spark boss said.