dec-feb = summer
mar-may = autumn (or the tedious 'fall', as you'd call it)
jun-aug = winter
sep-nov = spring

they aren't exact, sure, but it's tradition. august tends to be the coldest month (from a melbournian experience), despite being the 'lead-in' to spring.

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now lookee baby,
you're just nineteen.

Melbourne - COOL in the day, 15-16c max, <65F, even colder at night, not quite freezing but getting there, especially mid July.    

Sydney - Can be warm, gets up 70F at the warmest part of the day, but usually around 65F, won't freeze in the city but it will out west.  

Brisbane, Can get over 70F in the daytime but also freezes at night sometimes because of it's weird arse climate. Same goes for the Gold Coast.  

Anywhere north of the Topic of Cancer will be fucking sweet. 85F during the day and no colder then 65F at night. Because it's dry season in the Tropics, not very humid at all.

Watch out for all the little things. What you can't see, e.g hides in your shoes or in the bushes will kill you. Mostly spiders. Man-eating snakes mostly hibernate through winter, however that's different in the tropics, so make sure you're never without your rifle and frequency disturber.  DONT DO NEEDLE DRUGS IN SYDNEY.