An important part of the New Zealand immigration system to understand is an Approval in Principal for residency. When you get this you are close to getting residency, but you don’t actually have it yet. In fact it is possible for the decision to be reversed.
One thing you have to do when you get an Approval in Principal is pay the Migrant Levy. You also have to present your passport to a visa issuing office and, in some cases, produce other documentation. If you are an investor you will have to prove the transfer of funds. All these things can cause you problems if you don’t complete them quickly enough, or if your personal circumstances change in the time it takes you to complete them which makes you no longer eligible for residency. The best advice is to complete the final stages as quickly as possible.