The Commerce Commission has filed seven charges against Trustpower over the marketing of its bundled electricity and broadband offer.
The company's TV, web and billboard advertising between March and July 2015 promoted unlimited broadband data for $49 a month for 12 months.
However, the $49 price was available only to customers who signed up for power and broadband at the same address on a 24-month contract term.
For the second 12 months the cost of broadband would rise to $79, and if a customer wished to cancel the contract at any time during the 24-month period they would incur exit fees.
The second-year price was only disclosed in small print.
Trustpower has cooperated with the commission's investigation and has indicated that it intends to plead guilty to the charges.