Hello iisfaq, I used to work for NZ Customs, in fact I used to work at the Auckland International Mail Centre (where if you sent your package from overseas I'm assuming it was stopped)...
I was the firearms officer there so I only had a little bit to do with stopping of dutiable and uncustomed goods, I dealt more with the prohibited and forfeit goods, however...
From talking with the other officers there and being in the same office, with the same bosses I can tell you one thing, NZ Customs there is a JOKE..! Thats why I left for another agency, I couldn't take it anymore... Heres how a typical package stop seemed to work there...
Firstly most officers wouldn't stop a package with a declared value of under $400 dollars (the cutoff for a duty collection) unless they thought the items inside were prohibited (good officers unless they thought it was something serious or drugs), most would pick up something declared as fish food (or similar) off the x-ray belt and show the MAF officer (both are side by side), if they weren't interested, I wasn't interested... If it was under $400 and an officer stopped it and detained it under Customs legislation they were either; new or a jerk...
Then generally officers would come back into the office and look each other and say, "I think I should stop this what am I doing it under", then they would; ask other officers who had no clue, ask older officer who didn't care or would make something up and then STICK TO IT NO MATTER WHAT..!
You're right about the computer system, they were the first to move to a complete computer system in the early 90's..! And haven't improved it since..! Only adding more and more information to a crappy system, it is not user friendly (at all) and is practically useless for officers...
My advice when importing something: read the legislation, e-mail a customer service person from Customs (keeping the e-mail), when/if its stopped talk to the officer, if they are playing games ask for their Cheif Customs Officer's e-mail address, e-mail through the research you did, the e-mail from the customer service officer and hope they think you're not worth the headache...