I'm speaking as the female half of an older couple (Australian) who have just returned from driving in North America so I can understand your concerns about driving on the other side of the road. Like you, I really don't like guided tours - too easy to spend lots of time where you don't want to be and not enough time in a place that you really wanted to visit.
New Zealand is a perfect country for a driving holiday - easy to get around and the traffic is reasonably light. You can stop where and when you feel like it and if you avoid major holiday times, you won't need to book ahead for accommodation.
In Australia, the traffic around major cities like Sydney and Melbourne is not nice, so don't hire a car until you are ready to leave the CBD and get out into the country. You could always hire a GPS with the vehicle.
Don't come between Christmas and the first week of February - our long summer school holidays - that is, unless you want to be locked into accommodation and travel bookings well ahead of time. October/November (our spring) and March/April (our autumn) are generally good times to visit either NZ or Australia weather-wise. You will also find it easy to get accommodation without booking ahead.

A tour like kiwi experience could be a option, although 95% of passengers would likely be much younger then you but I been told a few older people do use it.

Its basically a bus which goes around a route, and you have the freedom to get on and off where you choice and stay as much time as you like. They do have designated overnight stops but in between there are plenty of short stops where people can depart or get back on the bus.

Ditto what marg says, but I'm an American living in Australia. The whole left side, right side is easily overcome (if I can do it anyone can), but best learnt far from an unfamiliar major city.

NZ is a great place to learn how to drive on the left - sparsely populated, not many large cities, easy to navigate...I reckon if you went to NZ first, you'd feel more confident to then drive in Australia, but then again, most Australian cities have wonderful public transport, so you could possibly forgo the car altogether and still not have to tie yourself down to a tour. You might pick and choose a few day tours here and there and perhaps self-drive outside of city centers.

We often visit NZ in the off season. Generally speaking, we've found the best off season NZ weather to be in April/May, October/November. Keep in mind that Easter is flat out busy in both NZ and OZ and that you'll want to work around local school holidays and "schoolies", which can be insane. Just google for specific dates.

Likewise, my favorite months here in Perth are April/May and Sept/Oct - after that it just gets too freaking hot for my Colorado sensibilities.

cusogolf, have you considered a private tour with a driver who is also your guide? There are people who do this sort of thing and they can match the tour to your interests and provide flexibility if things (weather, circumstances, expectations) are not working out. You could contact such local guides directly or use an agent who specialises in such tours.

Hi Cusogolf,

In my opinion, the factors that can make driving in a foreign country difficult are big freeways (USA), poor signage (Ireland), narrow roads (Ireland again), excessive speed cameras (UK) or a signage in a foreign script! (Asia)

New Zealand has none of these issues, and is basically a paradise for the self drive tourist.

Your only issue is getting used to driving on the left, and as other posters have pointed out, this is not a big deal!

I have some tips for this on my website, here: http://nzselfdrive.com/driving-in-new-zealand/

The main thing is to stay a night or two in NZ after your long flight here to rest up before taking picking up a rental (not an issue if you're in Australia first). And to have both the driver and front seat passenger taking responsibility and being concious of being on the correct side of the road.

Best of luck for your trip. Get in touch with me if you would like any advice and assistance with your planning~

In April the deciduous trees put on a great show of autumn foliage here in Otago, as well as in South Canterbury. The landscape becomes awash in golds, reds, rust colors. If you are thinking of traveling in 2013, the NZ school holidays are April 19 - May 6.
http://www.minedu.govt.nz/theMinistry/EducationInNewZealand/SchoolTermsAndHolidays/2013SchoolTermsAndHols.aspx
http://www.newzealand.com/sg/article/new-zealand-autumn-colours/

Apart from the school holidays, this would be shoulder season in terms of accommodation, so rates drop, plus there are fewer tourists than in summer. Though there are no guaranties, the weather tends to be more settled and not yet so cold in early autumn.

Here in coastal Otago, some of the migratory birds, such as Royal Spoonbills, leave for warmer climes at summer's end. Otherwise, autumn is a lovely time.

Regarding traffic, New Zealand only has 4.4 million people so, as others said, there aren't many people on the roads, especially in rural areas of the South Island and in more remote places on the North Island, such as the Eastern Cape and Northland. There are many winding, narrow roads on the South Island. Some are high roads that have sharp drop offs on one side. If you have driven the Hana Highway on Maui or California's Highway One through Big Sur, then probably these roads won't faze you. Frankly, if you drove in Italy, NZ will be a piece of cake.

Because the roads are generally one lane coming-one lane going, it is easy to get stuck behind a truck, caravan or other slower vehicle, but there are also frequent pass lanes--and you will be amply warned as to when they are coming up.

One hazard of driving on the opposite side of the road (of what you are used to) in a remote area (where there is little traffic) is forgetting where you are and accidentally drifting over to the other side. I noticed in the Catlins (south of Dunedin), that there were arrows painted onto the roadway, as if to serve as reminder for the tourists!

I think it is a great idea to fly into certain cities and take day tours out of these cities. For instance, you could fly into Queenstown, then take a day tour to Te Anau-Milford Sound Cruise. Or fly into Dunedin and take an all-day wildlife tour to the Otago Peninsula, and another day tour to the Catlins. Maybe try renting a car for a few days of your trip, for instance, from Queenstown to Lake Wanaka-Twizel-Mt. Cook-Lake Tekapo, or to Fox and/or Franz Josef Glaciers, or for just a day trip to the nearby Central Otago wine country.

Air New Zealand currently has a Grab a Seat sale. Jet Star is also worth checking for cheaper domestic and Trans-Tasman airfares.
http://www.airnewzealand.co.nz/
http://www.jetstar.com/nz/en/home
When you book Air New Zealand domestic flights in connection with your international Air New Zealand flights then you do not have to pay extra for your one checked bag. Otherwise, you pay extra for checked bags on domestic flights. A lot of the cheaper sale flights are non-refundable.

The Intercity buses are worth looking into.
http://www.intercity.co.nz/
http://www.coachnewzealand.com/busoperators.html
http://www.rankers.co.nz/tags/bus-and-coach

Rail links:
http://www.tranzscenic.co.nz/
http://www.railnewzealand.com/trains/