Car Rental recommendations requested - Australia and New Zealand

FYI, AutoEurope services both Australia and NZ, so you might start there to begin your research.
In Australia (Sydney, Melbourne and Brisbane) we wound up booking with Hertz but AE will offer a number of options to choose from.
I forget who we booked with in NZ (Auckland and Queenstown), but both rentals were pretty seamless and AE's customer support was good.
If I could offer my 2 cents: it's a very, very long drive from Sydney to Brisbane, along one of the most boring and least scenic highways in the country. It's a 2 lane highway too, which means you won't be making good time. Better (IMHO) to fly between the two cities a rent a vehicle upon arrival in Brisbane for your time there. There are also direct flights from Sydney to the Sunshine Coast airport near Noosa ... no need to go into Brisbane unless you want to drop the car there and catch one of their direct flights to Canberra.
Australia is huge and driving distances (and times) can be deceptively long - fly whenever you can. Qantas operates a budget airline called JetStar that we used quite a bit ourselves to get around the country. It's spartan but adequate for getting you where you want to go reasonably cheaply. Others here have recommented Virgin Australia but I have no personal experience with them.
There is a train connecting Sydney and Canberra - takes about 4 hours - and it was a pleasant way to go.

@BLM. Following is the view of an indigenous multicultural 2nd gen (boring Scottish/southern England heritage) Australian who has had experience in Canberra and northern NSW through Brisbane. And the Gulf of Mexico.

I enjoy the drive from Sydney, via the Hume Highway, through the rural southern highlands to Goulburn, and turning off on to the Federal Highway to Canberra. Beautiful countryside. The city is well laid out with many wide boulevard style roads and generally ample parking around the main sites, Museums, Parliament House, High Court etc. In the absence of your indicated time there, I also consider that a car would be more useful for Canberra and surrounds. Of course, depending on your reasons for choosing Canberra.

I find myself in complete agreement with Robert about the drive north from Sydney. Unless you are wanting to stop at some of the touted tourists’ attractions such as the much-publicised Byron Bay and its internationally known inhabitants. Or perhaps over the border at Surfers Paradise. Easy enough to fly to Sunshine Coast. Sunshine Coast, and Noosa in particular, will be popular around this time of year and may coincide with school holidays. Perhaps the reason for wedding scheduling. Arguably most stable weather before onset of humid weather and the cyclone season.

Domestically, I usually hire direct from international groups such as Budget, Avis, or Hertz, which are located at all the metro airports and the larger regional ones, rather than the smaller renters.

Regards Ron

@Robert and @Tassie Devil (Ron) -- I appreciate the details in both your replies. The info provides us with more food for thought. and thus we'll be able to make better plans that fit us personally.

FWIW, for >10 years, my friend and I annually drive from metropolitan Washington DC area to the western suburbs of Chicago (during Christmas time), and although the interstate highways for 4-lane divided (and not 2-lanes as @Robert wrote), many people (including my friend) say that going though Ohio and Indiana are very, very boring. Straight through, this is a ~12-hour journey, with stops usually about 13-14 hrs. We now break it up overnight, depending on when we start the journey into a 3-4 hr segment and then 8-9 hrs or vice versa. My definition of a "very, very, long drive" would be at least 15 hrs or more.

Here are two links that a travel agent provided re: the drive from Sydney to Brisbane. I'm sharing in case others are curious about what might be along the way.
4 days itinerary: https://explore.visitnsw.com/journey/d03f9076-fee3-11ea-96fe-067ec0c7e8f4/expert-guide-legendary-pacific-coast
3 days itinerary: https://explore.visitnsw.com/journey/b98c836a-f92a-11ea-96fe-067ec0c7e8f4/legendary-pacific-coast/default

Thanks again for the information about car rentals companies, Canberra, the Sydney-Brisbane drive, and more!

You have far more expert advice than mine but here’s my one time experience.

For the north island, I just searched for the best prices because car rental in NZ isn’t cheap. I ended up with Hertz through carrentals.com. But I rented in Dunedin and returned in Christchurch. Others have rented through Apex.

I rented with Alamo out of Melbourne and with Budget on the South Island. All were just fine to work with.

We visited Australia and New Zealand in October/November 2024. We always book through Kemwel/AutoEurope. In Australia, we used Sixt Car Rental for our first rental, and then later on we used Europcar. In NZ we used EZI Car Rental. We were very satisfied with every company we used.

You are wise to rent a car for this trip. My husband did all the driving. His only other experience driving on the left side was 2 years ago in Scotland, and he did fine. No problems at all.

I think if you have (if I interpret your post correctly) 5-6 days to travel from Sydney to Brisbane, you could make a pretty nice trip.
The highway has at least 2 lanes in each direction all of the way and isn’t usually congested unless there is an accident. The area just south of Brisbane sometimes has slow parts, in our experience, but not always. Of course you need to get off the highway to visit the nicest bits, but if you have 5 days, you will have a good amount of time for that. There are some very picturesque areas of coastline (I live on one) and nice small towns, along with national parks, lakes, etc.
I haven’t been from Sydney to Canberra by train - the regional trains in NSW are not luxurious but quite comfortable, particularly if you pay the small amount extra for first class. https://transportnsw.info/regional