The other thing worth noting is H&M, Zara and the likes aren't expanding quickly and are very unlikely to given the size and cost of their stores.
Worst case scenario they'll be in all the main centers in a couple of years, however New Zealand has strong population growth, a good economy and Hallensteins has far more reach across New Zealand than most clothing competitors.
I'm 25 and the truth is I wouldn't travel far for clothes. I'll either buy online, or where the most convenient store is. I think most other people in my age group would be the same (especially men).
I do think the uplift in Sylvia Park Hallensteins / Glassons will be largely because of the increased foot traffic for apparel in that area. I've heard about some of my friends who have gone to Sylvia Park to see H&M and have bought a few things from them and a few things from Glassons. Once the extra foot traffic dies down I'd be interesting to see if sales performance drops YOY in Sylvia Park for Glassons & Hallensteins, however I doubt this will have a significant effect on year end profit. It is also possible that the performance in Sylvia Park actually increases, but other stores experience a small decline.
The last thing I want to mention is I've been very impressed by Hallensteins marketing this Christmas. I've recieved several emails from them with great discounts to products. The product line seems a bit more fresh than it used to. They're hoping on board with Chinos ect and are very good and getting you to buy extra products. For example one pair is $70 but two is $100. This way they make a bit more profit overall, albeit at a lower margin.