Spark is facing legal action for allegedly overcharging thousands of customers.
The Commerce Commission is filing court proceedings against the telecommunications company.
The commission has laid 11 charges against Spark New Zealand Trading Limited (Spark) over alleged breaches of the Fair Trading Act.
These include incorrect billing for broadband data as well as for customers leaving the provider, and the incorrect rollout of a credit for new users.
Spark has already refunded more than $1 million to affected customers and donated more than $260,000 to charities.
Chief executive Simon Moutter said he was disappointed the commission did not accept a settlement payment to acknowledge its errors.