I agree, you should be comparing your paper 5 dollars with another one. In the standard catalog, the back of the 1992-97 paper 5 dollars is described as:
Brown and blue. Flora with Yellow-eyed Penguin at center.
Here is a picture of it:http://www.banknote.ws/COLLECTION/c.../NZL0177.htm
the back of the (19)99: (20)04: (20)05 polymer 5 dollars is described as:
Flora with Yellow-eyed Penguin at center.
Here is a picture of it:http://www.banknote.ws/COLLECTION/c.../NZL0185.htm
The colours appear to vary. So, I tried to compare your paper note with other paper notes I found on eBay here:http://www.ebay.com/sch/Paper-Money...3286.c0.m282
and the blue does seem to be more intense. However, you would need to find out how uncommon it is to have intense blue shading on these notes and then you may have something. You did right in holding on to it :)