The spider wasn't very big - only about two centimetres across, but it looked really mean. White tails originated in Australia and have been known in the North Island for over a hundred years.
A white tail doesn't build a web to catch its prey; instead it actively hunts other spiders. Its main method of hunting is to enter the web of its intended victim and mimic the struggles of a trapped insect. This tricks the resident spider into investigating the disturbance and so instead of gaining a meal, it becomes one when the white tail strikes.
White tails' bites are venomous, and they can and do bite humans. Typical symptoms of a white tail bite include pain and swelling at the bite site. The bite can develop into a small ulcerous wound that heals inside a week. In some instances, victims report flu-like symptoms.
There have also been reports in the media of people who have been bitten by white tails going on to develop more serious infections such as 'necrotising arachnidism', a severe form of skin ulceration, but these reports are generally unsubstantiated and it is not always confirmed that the bite was definitely made by a white tail. All the same, I'll try to steer clear of the little eight-legged darlings in future.
I was interested to note that The Australian Museum Online's factsheet on white tails mentions that
White-tailed Spiders around your house can be controlled by ... clearing away the webs of the house spiders upon which they feed.Up until now I've been leaving alone any spiders' webs I find, as they help to keep the number of insects down (we have a real problem with flies here) but this morning I went round with the long-handled duster and got rid of all the cobwebs.