If people from Dunedin do have a positive correlation between lead in their blood and criminal convictions, then I'd be asking why the convicts have more lead in their system in the first place.
From a quick google it looks like lead paint can cause higher blood lead levels. Perhaps the use of something like lead paint could be highly correlated to poverty. I know my gran has wallpaper on her walls instead of using paint - perhaps people in order to save money on their properties used lead paint instead of wallpaper. Perhaps state houses have more lead in them than richer family houses.
The point is without knowing much about this area, and from a quick scan of this article, it seems that they're trying to find evidence to support their current position of thinking around lead poisoning and crime.
I don't know if what I've mentioned above around housing is accurate at all, but it's a quick thought off the top of my head to demonstrate a potential for missing the forest for the trees, or an attempt to find 'evidence' for your preconceptions.