For those with a Tomtom GO 600 here's a problem description. The information is also available on the TomTom web site - if you look hard.

I noticed with the recent version upgrade to my TT GO 600 that the open road speed limits were showing as 80 kph instead of 100 kph. And when I installed the most recent upgrade that I hoped would fix the problem my entire NZ Maps were deleted leaving me with... well nothing! They were deleted without warning. Great when your 100s of kms away from home in unfamiliar territory!!

When I eventually found my way home and looked on the TomTom web site I eventually discovered that I needed to reinstall NZ Maps version 9.4 to get up and going again. I did this and appear to be working OK again. TomTom are apparently still working on a fix to the open road speed limit for version 9.5.