Three broadband providers, Slingshot, Orcon and Flip, have been fined a collective $121,500 for over-charging customers.
All three ISPs are owned by ASX-listed Vocus, the third-largest internet provider in NZ after Spark and Vocus.
The trio pleaded guilty to charging nearly 6000 departing customers a combined $132,000 (or $22 each) on their final invoice.
In sentencing in the Auckland District Court, Judge Glubb said the overcharging was "Not deliberate. Rather, it was a failure to implement and then ensure proper processes were operating. This was highly careless".
Slingshot, Orcon and Flip pleaded guilty and were convicted in relation to 13 charges under the Fair Trading Act for conduct that occurred between January 2, 2012 and March 1, 2018.
In a statement, Vocus says it "apologises unreservedly for its mistake." The company says it moved to immediately refund affected customers after the problem was brought to its attention, but anyone who want to check if they are due a payment can email email@example.com.
The case was brought by the Commerce Commission and forms part of a wave of filings that earlier caught Vocus' two main rivals.
A common theme has been people being charged for the remainder of a month after their final bill.
In March, Spark was fined $675,000 after pleading guilty to charges relating to misleading consumers in its customer invoicing and a $100 welcome credit offer to new customers.
The telco blamed technical glitches.
And in May, Vodafone was fined $350,000 for over-billing.
Vodafone chief executive Jason Paris said: "Every customer deserves an accurate bill every month. We clearly fell well short of that in this instance, and for that I apologise. If you get it wrong, you should put your hand up, acknowledge it, and make it right."