SQ B777 - Operating to Brisbane, Sydney, or Melbourne would usually mean carrying one of Brisbane, Sydney, or Melbourne as Alternate, and throwing in Adelaide as an optional extra.
Exceptions apply, for Brisbane for example, Rockhampton or Coolangatta could be considered, but considering that these airports offer only non-precision approaches and a fairly high minimum, are not often considered.
Planning with no Alternate at all is simply NOT an option.
Regards,
Old Smokey