Slang is a staple in Australian speech. The slang we use in Australia is unique - non-Australian native speakers are often both confused and amused by the way we speak. However, there are some patterns to our slang that can help you get by.
The most common form of slang in Australia is simply abbreviation. We're pretty lazy when we're talking and so we like to say things as simply and as economically as we can. Common examples of abbreviation slang include:
You can see that most of these words when abbreviated end in a long "e" sound, although many also end in an "o". These words are also more than three syllables long; the longer the word, the more likely an abbreviation for it will exist. The purpose of these abbreviations, essentially, is to make to words easier to say. Similarly, there are words that are actually made longer by slang because the longer form is easier to say, such as the famous Australian band ACDC which is colloquially refered to as "Ackadacka" because it rolls of the tongue more easily.
Another type of slang we have, which is less common, now mostly being used by older generations, is rhyming slang. So, if you are asked to pass the "dead horse" you should know that you are being asked to pass the tomato sauce. Likewise, if someone asks you if you want a "dog's eye", you are actually being offered a meat pie.
But my favourite type of Australian slang is undoubtably the use of similes. This type of slang is mostly used for entertainment purposes and to give emphasis. Unlike abbreviation slang which is all about ease and economy of speech, how we use similes is a very long-winded way of making a point that normally creates strong mental images and is amusing. Here are some classics:
flat out like a lizard drinking (very busy)
going like hot cakes (popular by demand - interestingly we don't use the term "hot cakes" otherwise, but refer to them as "pancakes")
mad as a cut snake (very mad)
stands out like a shag on a rock (really stands out, very obvious) as slow as a wet week (very slow)
This type of slang is perhaps becoming less popular. I think this is likely because (unlike the examples I gave) many of these similes tend to be rude and unrefined. So, you're more likely to hear this kind of slang from older people (50+) than younger generations, and also it's more common in the country than in metropolitan areas. However, these similes are still very much in use and gain much of their popularity from the fact that they are flexible. You can invent your own, and people do, much to the amusement of their friends and family.
So go ahead, give it ago. If you want to try blending in to the Australian scenery a bit, invent your own simile, and I'll be waiting here, eager as a kid on Christmas day, to hear it.