The Commerce Commission has laid 13 charges against CallPlus Services (trading as Slingshot), Flip and Orcon, alleging false representations in invoices.
The charges were filed in the Auckland District Court under the Fair Trading Act and relate to conduct during the period 2 January 2012 to 1 March 2018.
The Commission alleges that the Vocus-owned companies issued invoices charging customers for the period after the agreed termination of their contracts.
In doing so, the Commission alleges the companies misrepresented their rights to payments because their customers only owed payment for the services provided prior to the agreed termination date.
Mark Callander, chief executive of Vocus New Zealand, said the telco has been co-operating with the Commerce Commission during the investigation.
"Unfortunately, over a six year period, a very small percentage of customers leaving Vocus' residential ISPs who had given more than 30 days' notice of termination may have been billed incorrectly on their final bill," he said.
Vocus fixed this issue in early 2018.
"Vocus has written to all customers affected and offered refunds, and apologised for the mistake. People who think they may have been affected can email email@example.com."
This latest case of billing errors and misrepresentation joins a long list. Last April, Vodafone faced 27 charges. It was followed in July by rival Spark, also pinged for operational and billing issues.
Vodafone came back for more in August, facing 27 charges of misleading conduct.
Telco retailers have consistently led the Commission's league table for customer complaints.
The current case will be called in the Auckland District Court on 23 July 2019.