With only a week, you can hardly do justice to New Zealand. At most, you could visit one of the two islands. And, as you suspect, it will be much colder there than at the other destinations you propose, GBR and Darwin.

I've been to both the GBR area and Darwin several times. Either would be a great destination in August. If your co-worker is adamant about not returning to the GBR, then go to Darwin. With a week, you could easily visit Darwin, Kakadu National Park, Litchfield National Park and Katherine/Katherine Gorge. The Top End of the Northern Territory (Darwin et al.) is one of my favorite places in Australia.

Thanks Longhorn55! Would you prefer Darwin over GBR? My friend loved the GBR so he would return again in a heartbeat. We weren't sure how Darwin compared?

I don't know what you like to do or see, but considering: (1)you have never been to the GBR, the nearby Daintree Rainforest or Atherton Tablelands (2) your friend would "return in a hearbeat" and (3) you already will be closer to Cairns and therefore have more options in terms of flights and their costs (I would guess), I suggest go to the GBR and Daintree region.

A week is not a long time. You could spend it all or most of it in the GBR area. Also, if you never been to Brisbane before, there are many places to visit around there. You can drive up the Sunshine Coast all the way to Noosa, or visit the Australia Zoo, or hike in the Glass Mountains. Moreton Island is also close to Brisbane.

Here are some website links. Your local library no doubt has guidebooks on Australia; the photos might help you decide.

http://www.informationcentres.com.au/tropical-north-queensland/lp/Tropical-North-Queensland-Things-to-do-and-see.html

http://www.queenslandholidays.com.au/destinations/brisbane/brisbane_home.cfm

http://www.tourismtopend.com.au/

Leave New Zealand and/or New Caledonia for when you have more than a week.

I would go for Darwin, Kakadu, etc as longhorn has said. We were there late July 2011 and it was the most awesome weather for an awesome visit. I'd recommend all places mentioned as all have something totally different to offer. If you do go this way and are there on a Sunday or Thursday night do do do visit the Mindil Markets and watch the sunset on Mindil Beach. Pure magic. If you click on my name you should find my (very) long report from last year to give you an idea.

I would also recommend Cairns, but I haven't been in August - both our trips were late September/ early August and the weather was warm and mainly fine. The locals from the Cairns area can advise better on this area.

Good luck with wherever you choose - both places are worth it.

That's a tough question, Marstoad. The GBR and Darwin areas are like apples and oranges and I like them both!

The GBR region is going to be beaches and rainforests. The things we enjoy doing here are snorkeling on the GBR--a fabulous experience, relaxing on the beach, and exploring the rainforest (primarily with short hikes).

The Top End is a whole 'nuther world. The terrain is stark--red soil with scrub and large termite mounds. It's like nothing I have ever seen before and I found the scenery in the Top End amazing. In Kakadu, you'll find rivers populated with crocodiles and birds as well as huge rocks adorned with Aboriginal art. In Litchfield, you'll find lovely oases of natural pools which are great for swimming along with the amazing magnetic termite mounds. In Katherine, you've got two nearby parks (Edith Falls and Katherine Gorge) that offer natural pools, hiking, canoeing and great natural beauty. And Darwin is a neat town with some appealing attractions such as the aforementioned Mindil Beach Sunset Market and the Museum of the Northern Territory.

I think your choice depends on YOUR interests, so do a bit of research (reading guidebooks is my recommendation) to further see what's on offer in each place. You won't be disappointed no matter which you choose.

Lots of good advice and insight. Thank you all so much!

if it's August 2012 and you are going to Darwin the Darwin Cup is on then and accommodation may be a problem.
Darwin and ithe nearby areas are really worth seeing though.