Yep, with a TT system the impedance can be much greater, but why it's practically essential for all TT systems to have RCD protection for the whole installation.
As I understand from what Mike (Trumpy) has said in the past, all services in New Zealand are wired as TN-C-S (or what you might call the PEN system in Europe).
Personally, I'd be quite happy to see TT thrown on the scrap heap entirely. It's rather interesting to note that the American NEC doesn't allow TT earthing at all. Maybe that should tell us something?
