Australia is home to many unique and wonderful flora and fauna. It also has some iritating and in extreme cases dangerous pests. Heres a few notables:
 Cane Toads
Cane toads were introduced into Queensland as an attempt to combat a beetle problem in the cane growing areas in the far north. Unfortunately the introduced toads didnt tuck into the beetles as was planned but instead went on a rampage eating all kinds of local insects, fish eggs and native frogs. They've thrived in their new environment and have spread south to NSW and west as far as Darwin. They are regarded as a pest and many locals will invent perculiar and unpleasant ways of disposing of them.
Those keen on humane removal methods favour freezing them - but handling toads requires caution, they excreet a nasty powerful toxin from glands behind their ears which can spray if a toad is squashed. There have been cases of permanent eye damage from people getting toad secretions in their eyes when attacking toads with hammers, rakes, golf clubs etc etc
Australian crocodiles are an awesome powerful predator. There are 2 types:
Freshwater Crocodiles - live in creeks and river systems in Far North Queensland, NT and Western Australia, eat mainly fish and amphibians and are relatively small (<1.5m) - They are not considered dangerous to humans, but its probably still a good idea to avoid handling them!
Saltwater Crocodiles - Confusingly not confined to salt water at all and common all over the north of Australia in creeks, rivers, esturies, mangroves and even the open ocean and sandy beaches. They are feircely territorial and grow to huge sizes (5m+). The good news is that avoiding crocodiles is very simple - stay away from the water! Crocs are very fast ambush predators, but suffer from a very fast lactic acid build up so cant sustain any activity. You could outrun a croc on land easily (so I'm told)....
 Fire Ants
Fire Ants are a South American species of ant accidentaly introduced (probably via shipping containers) into SE Queensland. They are very aggressive and will attack 'en masse' giving you a nasty rash. (Pets have been known to be killed). If you have Fire Ants on your property then authorities will do their best to exterminate them but overall it may be a losing battle. SOuthern USA has given up trying to stop the spread of Fire Ants and SE Qld is desperately trying to erradicate them before they are so established its impossible. It may be too late!
Of course other ants still bite, and you need to watch out for big black ants which have a very painful bite - but its the 'group frenzy attack' of fire ants that make them so unpleasant.
In Northern Australia the summer is known as jellyfish season. Box jellyfish are have a clear squarish shaped body about the size of a tennis ball with tentacles up to 2m long. They are potentialy lethal and at best if you get tangled in them you're going to have some ugly and VERY painful burn scars. If you do get stung get out of the water immediately and seek medical attention (First aid is application of vinegar). Its almost impossible to see a box jellyfish in the water, so the best way to stay safe is to stay out of the water unless completely covered with a lycra 'stinger suit' or in a netted enclosure. Many beaches have warning signs up and vinegar bottles for sting treatment in emergencies.
Smaller and potentialy more dangerous than Box Jellyfish is Irukanji - which packs a more potent punch and is only about the size of a lentil. Often the initial sting is not felt but sickness comes rapidly over the next 5-120 minutesand very strong. If hospitalised quickly and proper treatment given recovery is possibel and you might get out of hospital after a couple of days!