I'm not really used to "thinking", but I will for a moment, and am happy to indulge ya.
It's quite a huge continent, there is much to see and do, and the distances from A-B are large. I'm presuming you'd be talking weeks (as opposed to months) - in which case, I tend to wanna tear my hair out when I think of giving advice on where to go and what to "see" with limited time -eek! Besides, we all have our limits to how long we can handle being on "holiday" and/or how much "sightseeing" we can take in one hit.
Port Douglas and the Great Barrier reef in Northern Queensland is a sight and such a treat:
http://www.youtube.com/watch?v=djxjYf5qHMU
http://www.portdouglasinfo.com/
But then I feel the same way about The Kimberley's in Western Australia
http://www.youtube.com/watch?v=gk4PL5vpzn8&feature=related
And The Great Ocean Rd, down in Victoria
http://www.youtube.com/watch?v=LPIA__hhYaQ
.....Ullaru/Ayers Rock (in the centre of Australia)
http://www.totaltravel.com.au/travel/nt/redcentre/uluruayersrock/directory/attractions
The deserts and outback with the hundreds and hundreds of wild running Kangaroos and Emus and the odd mob of wild Brumbies are pretty awesome too.
I'd want you to see and experience the lot, but of course, I know that there would be limitations, and that you'd have your preferences. Given that,I'd probably suggest the following:
Sydney. Then get a limo up to the Central Coast. You could hire a car once you're up here, but you wouldn't want to attempt driving through Sydney straight up - our road system is a nightmare for people who don't know it, let alone those used to driving on the other side of the road.
From here, there are various attractions.
http://www.centralcoastaustralia.com.au/AreaInfo/attractionsOverview.asp
(including the Australian Reptile Park)
http://www.hunterweb.com.au/wine/index.htm
http://www.visitnsw.com/area/Barrington_Tops_and_Surrounds/attraction.aspx
Then down in Sydney, there are several attractions
http://www.sydneyeguide.com/attraction.php
The South Coast (from Sydney to Melbourne), is just gorgeous - Ya could drive down with a couple of stop overs on the way.
Once ya get to Melbourne, ya could see the sites around there, and then take some time to go on the Great Ocean Road, with a couple of stop overs.
See the sites for what you're intersted in. Maybe an outdoor concert in the Hunter Valley at either the Bimbageon ot Tempus Two wineries - they are outdoor, and on with international performers from Spring through autumn/fall. Maybe a walk over the Sydney harbour bridge too, but ya have to book in advance. - All sorts of activities - depends what you're in to, and how fit and strong you are.
What's on the sites (including the croc shows, and handling the snakes at the Australian reptile park).
For the above, maybe 14.
Absolutely - the South Island of New Zealand is heaven on a stick.
You'd be best to take a bus tour - Fly into Christchurch, then a tour of Mt Cook, Lake Wanaka, and Queenstown. There are lots of activities along the way for those who are up to it.
Oh my, all 500 of us?
Awww, on a serious note, that's nice!

I'd have plenty of ideas, and that would depend on your budget. We are near water here, and have a jet ski and boat for water skiing wake boarding etc. If it was summer time, we could go to a particular park where there are a few jetties, tables and trees for shelter, then the sailing club at one end of it where they have a bistro - a nice spot on the water for dinner and drinks after a day out.
Whatever ya fancy. If we had notice, and you came up here, then I would enjoy taking a few days to show you around, and I could organise for a few others to join us for the odd outing, here and in Sydney.
Same.
Crikey, I got a bit carried away - Fair dinkum!