A lot. And in less than an hour at one stage.
There's some great photos here on The Age's website, and I was coming back from a meeting on the other side of town and took these pics.
Even where the road had sufficient camber for run off, the weight coming down meant there was about a centimetre plus of standing water.
And it was hard to see even with the wipers on top speed.
It didn't take much of a dip in the road to flood it.