Tiger Airways Australia, which has been grounded by the Australian regulator, is being investigated after two incidents in June involving Airbus A320s descending below minimum safe altitude.
One of its A320s descended below the published minimum altitude on approach to Avalon airport, Victoria, on 30 June.
The Australian Transport Safety Bureau said the incident occurred at 23:05 following a flight from Sydney. The aircraft involved was registered VH-VNC.
Investigators were already probing a similar incident on 7 June when a Tiger A320 (VH-VNG) operating from Brisbane to Melbourne descended to 2,000ft on approach despite the minimum safe altitude being 2,500ft.
Australia's Civil Aviation Safety Authority has grounded Tiger Airways Australia over "serious" safety concerns, but has yet to give a full explanation for its decision.
Tiger Airways had come under safety scrutiny earlier this year when CASA issued a show-cause notice to the airline.