As far as dressing for Australia is concerned, casual wear is generally the go.
If you wanted to, you could go to the opera in jeans, and no one would give you a second look, although this is one of the few activities for which some people like to dress up.
Unless it’s a really formal occasion, one has no need for a tuxedo or a formal long gown; a jacket and tie are not de rigueur for other occasions but the rule of thumb is usually whether one is comfortable with one’s choice of clothing for a particular occasion.
Some dress restrictions
Some clubs — such as returned services league (RSL) clubs or some sporting clubs — may have a dress code for general entry (for instance, no thongs, rubber shoes, jeans or collarless shirts allowed) or for entry to the club’s formal dining room (jacket and tie required). These rulings may vary from club to club, and one usually has to be signed in for entry.
If you plan to visit any of Australia's casinos, such as Star City in Sydney or Wrest Point in Hobart, jeans and other casual wear — except really scruffy ones — are certainly acceptable.
And do dress for the weather, of course.
If visiting Sydney, here's the average city temperature range:
December: 17.5°C (63°F) - 25°C (77°F)
January: 18.5°C (65°F) - 25.5°C (78°F)
February: 18.5°C (65°F) - 25.5°C (78°F)
March: 17.5°C (63°F) - 24.5°C (76°F)
April: 14.5°C (58°F) - 21.5°C (71°F)
May: 11°C (52°F) - 19°C (66°F)
June: 9°C (48°F) - 16°C (61°F)
July: 8°C (46°F) - 15.5°C (60°F)
August: 9°C (48°F) - 17.5°C (63°F)
September: 10.5°C (51°F) - 19.5°C (67°F)
October: 13.5°C (56°F) - 21.5°C (71°F)
November: 15.5°C (60°F) - 23.5°C (74°F)