I was spreading mulch (by hand - no gloves) when I came across one of Australia's most dangerous spiders - the Red Back Spider. She was feeding on the green praying mantis (to the left of the photo).
The female Red Back Spider has the red stripe down her back and can live for 2-3 years. The male is much smaller, no red mark on his back and only lives 7-8 months.
The bite is highly venomous and characterised by intense localised pain and around the bite site. Other symptoms may include sweating, muscular weakness, loss of co-ordination and, in severe cases nausea, vomiting, convulsions, etc.