I was so confident yesterday that I had sorted everything out. I spoke too soon.
When I went back and re-read the page from Google, I realised that it wasn't that my blog was now on blogspot.com.au, it was that it existed on both domains and as an Australian reader of the blog, I would (randomly?) get sent to blogspot.com.au.
So the fact that I migrated all the comments to the .com.au domain blog didn't fix the problem. The redirect wasn't "flaky", it was ill-advised and a really crappy solution. DISQUS are looking for a way to deal with it, but I can't imagine there's a simple, sensible solution, and anyway, I'm not being redirected today so maybe Google have had problems and reconsidered.....
I found this blog this morning that has more of an explanation. I'm currently migrating the comments back to the blogspot.com domain and seriously considering migrating the whole blog to wordpress. :(