Behodar: In my experience, the USPS tracking site is best while the package is in the US. Once it arrives in NZ, NZ Post's site tends to be more up to date. In one case I've had the USPS site say "arrived in NZ" two days after NZ Post did.
Short version: For the latest details, put the tracking number into the NZ Post site.
Yeah that's where I've been watching it from, still has the same info though.
1 Picked up 05.49 AM on 25/01/12
2 Left country of origin 09.34 AM on 26/01/12