Does formal attire mean long dress for girls in Australia?
On Princess, not at all. Here, "formal" means anything from cocktail attire, to nice dress you could wear to an afternoon wedding or out to dinner, to black pants and sparkly top combos. Generally, you see all three in about equal numbers. There are usually very few long gowns, mostly on older women/prom-aged teens.

Formal wear, formal attire or full dress is the traditional Western dress code category applicable for the most formal occasions, such as weddings, christenings, confirmations, funerals, Easter and Christmas traditions, in addition to certain audiences, balls, and horse racing events. Formal attire is traditionally divided into formal day and evening attire; implying morning dress before 6 p.m., and white tie (dress coat) afterwards.[citation needed] Generally permitted other alternatives, though, are the most formal versions of ceremonial dresses (including court dresses, diplomatic uniforms and academic dresses), full dress uniforms, religious clothing, national costumes, and most rarely frock coats. In addition, formal attire may be instructed to be worn with official full size orders and medals.
With background in the 19th century, the protocol indicating particularly men's formal attire have remained virtually unchanged since the early 20th century, and remains observed so in certain settings influenced by Western culture: notably around Europe, the Americas, and Australia, as well as Japan. For women, although fundamental customs for ball gowns (and wedding gowns) likewise apply, changes in fashion have been more dynamic. Optional conventional headgear for men is the top hat, and for women picture hats etc. of a range of interpretations. Shoes are often dress boots. Short evening gloves may be worn.
"Formal attire" being the most formal dress code, it is followed by semi-formal attire, equivalently based around daytime black lounge suit, and evening black tie (dinner suit/tuxedo), and evening gown for women. The lounge suit and cocktail dress in turn only comes after this level, traditionally associated with informal attire. Notably, if a level of flexibility is indicated (for example "uniform, morning coat or lounge suit", as seen to the royal wedding of Prince Harry and Meghan Markle in 2018), the hosts tend to wear the most formal interpretation of that dress code in order to save guests the embarrassment of out-dressing.
Since the most formal versions of national costumes are typically permitted as supplementary alternatives to the uniformity of Western formal dress codes, conversely, since most cultures have at least intuitively applied some equivalent level of formality, the versatile framework of Western formal dress codes open to amalgation of international and local customs have influenced its competitiveness as international standard. From these social conventions derive in turn also the variants worn on related occasions of varying solemnity, such as formal political, diplomatic, and academic events, in addition to certain parties including award ceremonies, balls, fraternal orders, high school proms, etc.