You can either take a very short flight from Invercargill to Stewart Island, or you can take a much longer ferry. If your time is limited on Stewart, I would recommend maximising your time there and not on the journey-- Take the plane! The airline is nice, easy to use, and not terribly expensive. The flight takes about 30 minutes. The plane is small (about 10 passengers) and crowded. It can get pretty bumpy (this area is known for their high winds), but the airline and the plane is safe. If you are prone to airsickness it might not be for you, but then I suspect you would also be prone to seasickness and the ferry would be no better- probably worse because you would feel sick longer. It is also nice to see the view from the air to get a broader perspective on the area.

Chose to fly for the 15 minutes, rather than do the boat ride....

Well, what an experience, didn't seem to be flying too high so visibility was excellent. The whitecaps and the wind coming across us combined with the odd bump made for an interesting flight.

I've done the flight from Katmandu toLukla and this one was in the same league... All good though.

Looking forward to the return flight on Sunday....

Decided to take the flight to Stewart island rather than the ferry. We booked well in advance as we needed to be on the Island at a particular time. Booked in at Invercargill Airport after leaving the hire car in the car park. Plenty of spaces. The airport is very new and pleasant with a small cafe available for drinks and snacks.
Didn't have to go through any security. Luggage weight is limited so we only took backpacks. The plane only takes 9 passengers and is very small and somewhat elderly. The pilot was a young man who definitely enjoyed flying the plane. It was a very windy and wild trip over and although only 15 minutes it was great. We had a good view of Invercargill and Stewart Island. The pilot expertly landed the plane despite the windy conditions. We landed on a tarmac strip in a field. A minibus then picks you up along with your luggage and delivers you to the centre of Oban.
On our return you book in at the Post Office to await the minibus. Going back I asked the pilot if my wife could sit next to him as she loves planes. He agreed and she had a lovely trip back. Great experience and, in my view, a better bet than the long ferry journey but not for the nervous types!