It basicly a race where you do as many laps as you can in 12 hours, you can go solo or in teams of 2,3,4,6 and mixed male/female.
more info here http://www.wa12hour.com.au/
its pretty good, atmosphere is great, the commentary goes on all night over the PA system, theres a coffee van going all night too!
The one problem i have found with the 12 hour is not much goes into advertising, so the numbers have been dropping over the years, the guys that run it do it volenteer their own time, so without them, the event wouldnt happen, so it would a huge loss if it were to be cancelled due to low numbers.
Steve