I wouldn't say oversupply is massive in all areas. I think we have well over built and well under built in different areas (under building is driving legitimate shortages in some Sydney areas).
Ultimately even if we are 400,000 houses over the mark (and it's likely the number is less than that, the table doesn't account for everything) that's only around 5% over the mark (400k/8m total).
I wouldn't say it's definite proof of oversupply, but I think it's a good measure to show that the "shortage" is a myth. A lot of commentators have referred to the gap (between pop growth and new housing) over the last 4 years when talking about the shortage without considering what came before it.
There was an oversupply in the US before their bubble collapsed, what kept prices up there?? The answer has been posted on this forums by Sunfish (and evand if I recall) several times in the last few weeks, it's not fundamental drivers that have pushed the markets to these heights, it's availability of credit and sentiment.
The oversupply/undersupply argument is an interesting one, because on a national level the numbers don't have a lot of relevance (at least not to me). I'm sure we can all name places where there are properties lying vacant, with noone to live in them. Conversely, we can all name markets where any new stock gets snapped up faster than you can blink.
The real question, I think, is what is the trend in population growth movement, and is there a genuine oversupply or undersupply in the key areas?
Given development has slowed down, but population is increasing, it's only a matter of time until demand in key areas grows again.
The those vacant houses in the middle of nowhere will still be vacant.