On several AIX 5.3 LPARs the timezone is currrently set to:
TZ=NZST-12NZDT,M9.5.0/02:00,M4.1.0/03:00
Daylight savings in NZ starts on Sun 28th Sep 2008, which is the fourth Sunday. Do I need to change my TZ variable to NZST-12NZDT,M9.4.0/02:00,M4.1.0/03:00 or will AIX interpret the fifth Sunday (my current timezone setting) as the last of the month and change on the 28th?
Exactly this is the meaning of the "fifth" weekday: the last in a month. Maybe this will provide some additional information, but you seem to have figured it out pretty well already.