Well worth the side trip if you plan on travelling to Stewart Island from Christchurch or Dunedin. Then on to Queenstown via Doubtful Sound or Milford Sound. I would suggest you take the short detour via the Catlins Coast on the lower South East Corner of the South Island.
Not only are there some of the most spectacular coastal vistas and primordial rainforest views but you will get to experience some of the worlds most unique& rare wildlife, up close and personal. Off the beaten tourist trail but less than two hours drive from Dunedin and one hour and a quarter from Invercargill, you will realise instantly why they call this "The Forgotten Coast".
I suggest you check out the website: www.catlinsdiscoveries.com for a free map and some excellent photography ideas of what there is to see and do in the Catlins.