Eleven charges have been laid against Spark by the country's competition watchdog for allegedly misleading and overcharging customers.
The charges arose from three separate incidents between June 2014 to December 2017.
The Commerce Commission alleges Spark overcharged customers for broadband data when a fault in Spark's broadband network misrecorded customer data usage.
The telco sent letters offering new customers a $100 account credit for subscribing to a particular broadband plan but failed to mention the offer could only be redeemed by phoning Spark, the Commission says.
The offers allegedly created the impression that customers signing up online would receive the credit, when they wouldn't.
And from June 2014, Spark's terms and conditions said charges would stop 30 days after the customer gave notice to terminate their contract.
However, the Commission alleges that a customer's final bill included charges for the entire next monthly billing period regardless of when the Spark service stopped.
Spark says they have already applied credits to the accounts of all impacted customers.
In May, Spark announced it was attempting to refund $1.1 million to former customers.
Spark’s managing director Simon Moutter says "These were all system-based errors caused by genuine mistakes with no malicious intent involved on the part of Spark. That being said, we are deeply disappointed that these issues have affected our customers".
"Our preference has consistently been to settle these matters and avoid court proceedings.
"To this end we have held extensive discussions with the Commerce Commission, including our suggestion that we make a settlement payment (possibly in the form of further charitable donations) to acknowledge our errors."
Spark says the 2015 equipment fault issue has been resolved for all 5,325 affected customers who were overcharged in total by $216,937.
The 2016 'welcome credit' affected 463 customers who have been recreditted $46,300.
The 30-day billing issue affected a number of customers who left Spark over a three-year period, most of who have been refunded, Spark says.
As of last week, there were up to 8,829 former customers, owed a total of $304,070.
More than 90 percent of those former customers are owed less than $100.