Jetstar and Virgin Australia misled customers about ticket prices on their mobile sites in 2014.
The Federal Court has found the airlines' use of "drip pricing" was misleading, deceptive and contravened Australian consumer law.
"The ACCC's concern with drip pricing has always been to ensure that consumers are not misled and that businesses are not unfairly disadvantaged by misleading practices," Australian Competition and Consumer Commission chairman Rod Sims said on Tuesday.
Drip pricing is where an advertised price becomes unavoidably higher as the online transaction progresses.
The ACCC said Jetstar and Virgin failed to adequately disclose an additional "booking and service fee", which was charged on bookings paid using most credit cards or PayPal.
A Jetstar statement said the company wanted customers to understand all fees and charges associated with bookings.
"That's why we have progressively made changes to make it clearer at every step of the booking process what charges may apply," the company said.
Jetstar said millions of flights were booked without fees each year, using direct payments, the POLi system and Jetstar's own credit cards.
