The Australian Redback is a very small but very dangerous spider native to Australia. Along with the Australasian funnel-web spiders, the redback are the most dangerous spiders in Australia. It has a neurotoxic venom which is toxic to humans with bites causing severe pain.
Considering they are only as small as a dime, they are sometimes hard to see - but don't worry - they will certainly let you know about it if you disturb them. :)
I almost stepped on this one with my bare feet while I was walking around my backyard (it's summer here, so you tend to walk around with bare feet). Luckily I didn't disturb it - well, at least not until I finished admiring it. :)