Qantas Airways Ltd. was served with a class-action lawsuit by lawyers who allege the airline failed to refund passengers for flights cancelled during the pandemic and illegally benefited by retaining billion of dollars of customer funds.

By instead issuing flight credits with strict conditions of travel, the Australian airline enriched itself with interest-free financing at customers’ expense, according to a lawsuit filed in Federal Court last week by Echo Law and served on Qantas on Monday. Sydney-based Qantas didn’t respond to a request for comment.

Fares have soared since Covid travel restrictions eased, forcing customers to pay more than the value of their credits for fresh flights, said Echo law partner Andrew Paull. The airline has pressured passengers to redeem their vouchers or lose the credits entirely, Paull said. The lawsuit is seeking refunds for all remaining credits and compensation for the years that customers have been out of pocket.

The claim raise questions about the function — and legality — of travel credits lingering on airline balance sheets around the world. Qantas is this week set to report record annual profit after demand for travel jumped following the pandemic.

Read More: Qantas Sees Fatter Profit Margins as Covid Revamp Kicks In

Qantas racked up some A$2 billion ($1.3 billion) in Covid-related travel credits. Last month it encouraged passengers to use up the remaining A$400 million of credits, offering double the normal number of loyalty points for bookings made with the vouchers before the end of July.