Casual wear is generally the way to go when you're visiting Australia. You can go to the opera in jeans and no one will give you a second look, but this doesn't mean that everyone else will be wearing jeans, too. Some activities in Australia inspire some people to dress up.
"Formal" Wear in Australia
No one needs a tuxedo or a long, formal gown here unless it's a really special occasion. A jacket and tie are not de rigueur for even less formal occasions.
The rule of thumb is usually whether you're comfortable with your choice of clothing for a particular occasion. In most cases, jeans can be your wardrobe staple -- you can dress them up or down depending on where you're going. You might want to pack some non-denim casual wear if you plan on visiting city restaurants, but you can leave the dressy clothes at home.
Some Dress Restrictions
That said, a few places have dress restrictions. Some clubs, such as Returned Services League clubs (RSL) and sporting clubs, have dress codes for general entry. No thongs, rubber shoes, jeans or collarless shirts are allowed for entry to the club’s formal dining room. A jacket and tie are required. The rules can vary from club to club and you must usually be signed in for entry, so check ahead with the place you plan to visit to be on the safe side. You don't want to arrive only to be turned away.
If you plan to visit any of Australia's casinos like Star City in Sydney or Wrest Point in Hobart, jeans — except really scruffy ones — and other casual wear is certainly acceptable.
Of course, you'll want to dress for the weather, too. Temperatures in Sydney range from the mid-forties to the lower fifties in winter, and from the upper sixties into the seventies in summer. Remember, summer months are December through February in the Southern Hemisphere. Winter is marked from June through August.
If you're visiting an area that's notably hot in summer, consider packing lots of clothing made with natural fibers. Don't forget sunglasses and a hat to help protect against the glare of the Australian sun.
Here's a summary of what you can expect temperature-wise. As far as rain, snow and other weather events go, these links can provide more information.
December: 17.5°C (63°F) to 25°C (77°F)
January: 18.5°C (65°F) to 25.5°C (78°F)
February: 18.5°C (65°F) to 25.5°C (78°F)
March: 17.5°C (63°F) to 24.5°C (76°F)
April: 14.5°C (58°F) to 21.5°C (71°F)
May: 11°C (52°F) to 19°C (66°F)
June: 9°C (48°F) to 16°C (61°F)
July: 8°C (46°F) to 15.5°C (60°F)
August: 9°C (48°F) to 17.5°C (63°F)
September: 10.5°C (51°F) to 19.5°C (67°F)
October: 13.5°C (56°F) to 21.5°C (71°F)
November: 15.5°C (60°F) to 23.5°C (74°F)