If it's an international op, you need prior approval from CASA
From the Australian AIP:
2.3 Alternate Airports to International Airports
Avalon
Customs, Health and Immigration are provided
to coincide with approved flights only. Avalon
Airport must not be nominated as an Alternate
unless approval has been obtained by the operating
airline from the Civil Aviation Safety Authority
Albury, (as suggested by Pyro), is not a widebody airport !