I'm heading down again this year (wouldn't miss it for the world). People from every cycling subculture come together for a day to pound the pavé of the inner north in a crazy carnival of orienteering, dress-up and laughs. The ride itself is about 60-70km (including getting to the start/home from the end/crazy detours/stuff it let's go to the pub for a bit), but there are no prizes for getting to the end first - you'll miss most of the fun if you do.
I can't recommend it highly enough. It was amazing last year even with the rain (for the uninitiated, it doesn't rain in Melbourne like it rains here - it's considerably less heavy and usually fine to ride in). Here are the last two I did on Strava:
https://www.strava.com/activities/159357097
https://www.strava.com/activities/62237332
And some of the pictures from last year from the Fyxo site (that Duffman costume was waterlogged even before the start - amazing he's still wearing it at the end):
