From what I see (from the page sources) it appears that msn.co.nz is measuring traffic using Nielsen NetRatings, but that Yahoo!xtra is not. (I may be wrong).
This will make things very “interesting” for Yahoo!xtra advertising sales teams. Corporate Yahoo! may not realize that the advertisers in NZ demand tracking from NetRatings, and the locals will pay dearly if they do not join the club.