An Australian air safety company has revealed that Indonesia's Yogyakarta airport was operating without a license when a Garuda Indonesia plane crashed last year, killing 21 people.

Flight Safety Pty Ltd, which carried out an audit of the airport following a request from an unnamed client, says authorities had failed to implement five conditions for a license - including extending the runway and safety area.

The Indonesian state carrier's Boeing 737, with 140 people on board, bounced and skidded off the runway in Yogyakarta before bursting into flames in March last year.

An Indonesian safety official denies the airport was functioning without a license, but agrees that the runway end safety area was not long enough.

Flight Safety audited Yogyakarta, Solo and Semarang airports, and found all three airports were, at the time, operating without a licence.