New Zealand

I don't know anybody who has visited NZ without renting a car or taking an organized tour so I can't really comment on what you can do or where you can go by public transportation. There is good public transportation in the major cities (Auckland, Wellington, Christchurch), and buses between cities, not sure about getting out and about. When I was planning my trip I found a wealth of helpful information on their main tourism website. You can find lots of options for day tours or multi-day tours, etc.

Although I spent the majority of my time in NZ on a group tour, I did fly in a couple days early and I used the Intercity bus to travel from Auckland to Waitomo to spend a couple days in the countryside and do a glowworm cave tour. Then I took the bus back to Auckland to meet up with my group. I bought the tickets ahead of time online (www.intercity.co.nz) because it was cheaper to buy ahead and I knew my exact travel days. It was very easy and the busses were nice. They even had free wifi which I did not expect.

I have no experience with public transport in Queenstown, but I know that some of the day tours/trips have offices right in the middle of town where you can be picked up/dropped off and Queenstown itself is very walkable. So depending on your interests you might be able to get by with that. I did a day trip from Queenstown (not part of my tour) that included jet boating and a 4 wheel drive tour and was picked up at the tour office and dropped off back at my hotel at the end of the day.

I think if you're looking for more freedom and the ability to go wherever you want, a car is probably the best option. But the busses, at least on the North Island, can be useful if you're willing to go where they go and stick to their schedule. You could check out the schedules online to see the various options.

While renting a car would give you the greatest flexibility, are you comfortable driving in winter conditions? Queenstown, in particular, can still get periods of heavy snows at the end of August. While I traveled in New Zealand during the tail end of summer and into the Autumn months, I do recall the driver of one of the hop-on/hop-off backpacking buses I was on pointing out that in winter on the South Island in the area around Queenstown, there can be times when some roads are completely impassable due to snow rolling down onto the surrounding roadways - so a local driver/guide may be best suited to know the most efficient ways around things like that.

If the answer is no you are not comfortable driving in winter conditions, then definitely look into getting a private tour guide or working through the tourist info offices to get info on day tours that will include bus transportation.

Why are you including Queenstown? I ask because you mentioned your age as a possible barrier to car rental. I'm assuming that means you won't be participating in winter sports? Queenstown is a very youth-oriented place, with plenty on offer to the adventurous, all year round. If you're not going to be skiing, bungy jumping or riding in a jetboat, that leaves sightseeing, for which you will most definitely need a car - or rely on tours. I'm not sure how many non snow-related tours will be operating in August though. I think the Milford Sound tours will be running and they are highly recommended. Just be sure to wrap up warm for your time on board the boat.

It will be scenic, that's for sure.

If you're not completely wedded to the idea of Queenstown, I would consider staying in or around Auckland, which will be a tad warmer. From there, you can visit some lovely country & coastline, or head south to the volcanic region around Rotorua. Auckland itself I find a bit bland.

August is a great time to visit Australia! :-)