I would start looking at 3-4 weeks. Whether genetically or because they were born first, the ones that have the stocky body and good confirmation, will likely be the same ones that meet that test latter. The ones that appear un-thrifty at a young age tend to be those that keep those characteristics. After they go though the "skinny" stage (fat belly, narrow shoulders) which starts at 5-6 weeks, they will begin filling back out at 10-12 weeks. If you can individualize your kits by sight (this might be hard with NZW) or keep them non-intrusively marked, it would be good to follow a whole liter from when they come out of the nest box until about 12 weeks. You will be able to get an appreciation for your particular breed and strain, so that the poorer animals can usually be identified early. That way you can get an idea of the best at 4 or 5 weeks and be able to process most of the litter at your preferred processing age, while keeping the rest till they are more mature. I don't show any more, but I still judge for "breeders". The build of an outstanding animal will often "reach out an grab" one as early as 4 or 5 weeks.
If you raise non-white animals, you can check early for disqualifications such as miss-colored nails, etc. I have always found ears to be the hardest thing to evaluate early. In my rabbits, it seems ears are the last thing to mature into their permanent form. That is not a problem in my herd, because I am selecting strictly for vitality and meatiness (heh, and for broad heads, which I shouldn't do) :?
Actually, I have achieved the health/vitatlity and meatiness standards which I set, and I am starting to throw in another parameter when all other things are equal. I find that those kits that come out of the nest early, are curious and less timid, tend to make much better animals. They not only are much easier to deal with as breeders, but they make much better "family rabbits" if I sell them.
Another thing to always check before actually designating canidates-still-in-running is feel. With (meat) show rabbits, that occasional one that you pick up that feels "solid" or like a block in your hands is usually superior....All confirmation being the same, this "solid" feel will knock the socks off of a judge at a competition.
Obviously my comments don't necessarily apply to "fancy" rabbit breeds.