The Australia Transport Safety Bureau (ATSB) is investigating a 30 July incident in which a Virgin Australia Embraer 190 flew below the minimum safe altitude while approaching Essendon Airport.
The aircraft, with registration VH-ZPA, "was about 700 feet [213m] below its assigned altitude", said ATSB.
The aircraft was flying from Sydney to Melbourne.
ATSB confirmed that there were no injuries, and it classified this incident as "operational non-compliance".
ATSB is still conducting its investigations and expects to issue its final findings in December.