Redbacks are considered one of the most dangerous species of spider in Australia. The redback spider has a neurotoxic venom that is toxic to humans with bites causing severe pain, often for over 24 hours. An antivenom is commercially available, and since its introduction in 1956 there have been no deaths due to Redback bites.
Black Widow Spider, poisonous and dangerous. (also upside-down) << This spider actually seems to be a Red Back spider with the markings on it's back. They're easily confused with Black Widows due to the extremely similar coloring.