The Commerce Commission is investigating the use of ''opt-out'' pricing on the Jetstar airline's website.
The online booking service automatically ticks the baggage box, which adds $11 to the price, unless the customer deselects it.
Jetstar.com offered pre-selection on three products during the booking process: checked baggage, seat selection, and travel insurance.
It said preselected were clearly displayed and easy for customers to deselect by ticking a button and continuing with their booking.
Jetstar said it believed the process complied with all laws, and it was communicating with the commission about it.
Air New Zealand was warned last month over the same issue.