Three telecommunication companies have been fined $121,500 for continuing to charge customers after their contracts ended.
Slingshot, Orcon and Flip, owned by Vocus Group, plead guilty to charging nearly 6,000 customers after they terminated their contract.
Among the terms and conditions, the companies said they would stop charging customers one month after they gave notice ending their contracts.
Auckland District Court Judge Kevin Glubb said the actions didn't appear deliberate but failed to implement and ensure proper processes were operating.
"This was highly careless," Glubb said.
Commerce Commission chairwoman Anna Rawlings said the organisation expects businesses to have processes in place to ensure their invoices are accurate and compliant with what they tell their customers and with the Fair Trading Act.
"In this case, each company failed to take necessary steps to ensure they were. As a result, they misled and overcharged thousands of customers."
Vocus has since issued a statement saying they have fixed the issue.
"We had quickly developed a software solution to fix the mistake shortly after the Commission brought the issue to our attention and commenced contacting customers to issue refunds," it said.
"Whilst we were surprised the Commission decided to still take the matter to court, we have accepted our mistake. The issue occurred in some instances when people gave more than 30 days' notice of leaving, which required Vocus relying on a non-automated process to properly issue the final invoice.
Vocus said they would like to hear from anyone who hasn't claimed their refund or would like to check whether they are due a repayment.