Redback Spiders (Latrodectus hasselti) are very common in Australia. They are even kind of popular: one Australian beer is named after them.
Redbacks can be found everywhere in Australia, especially in the densely populated urban areas. They thrive in the warmer regions. I've seen hundreds, mostly on verandas, in sheds, in storage yards, on industrial sites and inside houses. They also hide in hollow logs, wood or junk piles etc.
The Redback Spider is related to the venomous Black Widow Spider and looks very similar.
The only difference is the red back, or rather the very distinctive red dorsal stripe that you can see in the picture.
The size of a female redback spider is around 1 cm for the body (males are tiny). You can see the body is big in relation to the legs.
Their webs are very distinctive too. They are messy and always seem to contain several egg sacks, white to creamy coloured oval blobs, 1 cm or bigger in size.
Redback spiders love to hide in sheltered spots, holes and crevices. I usually notice the web before I see the spider.
I don't know of any Australian spiders that make stickier webs. They are hard to impossible to wipe or brush off and the vacuum cleaner is totally useless.
Small insects make up the bulk diet of Redback Spiders, but they sometimes kill and eat much bigger prey, even small lizards if they get tangled up in the sticky web. They also steal wrapped up food items from each other's webs if they can.
How dangerous is the Redback Spider?
Redbacks are responsible for the vast majority of serious spider bites in Australia. However, they are not considered dangerous.
Only the bite of a female Redback Spider is toxic (the males are too small to bite anyway).
To get bitten you have to actually stick your hand into the web of a spider, they rarely leave their nest. The fangs of the Redback Spider are tiny. Even if you do manage to get bitten the bite is likely ineffective. In addition the Redback Spider venom is a very slow acting toxin, and most people don't show any reaction to it (except it itches like crazy).
Possible symptoms in those who do react are pain (can become severe), localised sweating at the bite site, and later on more sweating, muscle weakness, nausea and vomiting.
A simple ice pack is the best first aid. In most cases it's all that's required as very few people actually develop these symptoms (about 6% of those bitten, to be precise).
Honest, if you go and see a doctor here and tell them a Redback spider bit you, they'll probably just tell you to go home and put ice on it. (At least that is what happened to a work collegue.)
A pressure immobilisation bandage as used for snake bites or funnel-web spider bites is not recommended. If you do develop severe pain or start feeling sick then you should of course go and seek medical help.
Antivenom is available and there is a lot of time (days!) to administer it. Fewer than 1% of those bitten will require antivenin, and nobody has died from a Redback Spider bite since the antivenom was introduced well over 50 years ago.