I logged into the NZIS site to check the status of our PR application and found that they said this:

Health Certificate Details:
Below is the status of your most recent Immigration Health Certificate:

Certificate Type Date Received Status: Received
Full Medical 24 Sep 2009

When your health certificate status has expired, Immigration New Zealand will request another health certificate from you.

Note the bold which has gotten me worried--do we need to do it AGAIN after it has expired (which is 3 months, right?) I thought that once you had your medicals in with the residency application, that was it. Either I'm going crazy or their phrasing of this is very confusing.