How exciting to be on the verge of a trip. We have just come back from four weeks in South Africa and I took my tripod with me and it never came out of the bag. I know you will be travelling around Australia, so this may be a different scenario for the landscape photos, but the only time I have ever used my tripod is on New Year's Eve for photos of fireworks over the Harbour Bridge. I am a Canon girl and I took my Canon 18-200 lens and my Canon 100-400 L series lens with me. The 18-200 stayed on the camera when out and about, but when we went to Kruger and Pilanesberg, the 100-400 was on the camera the whole time. It was a little restrictive when the animals were close to the car, but for long distance photos it was great. You can pick up 100-400 lenses on fleabay (I love that - my new term for on sites that don't like advertising). They are fairly cheap these days, but if you don't think it will be of use after the trip, the 55-300 will be fine. On my trip in 2007 I had a 75-300 lens and it was okay for most safari shots. I did have a Canon 10-22 wide angle, but found I didn't use it much, so I sold it (it did take beautiful wide angled photos though). I found the 18-200 does me for the occasions that I want a wide angled view, so I am sure that your 18-105 will be fine as well. It really depends on what you will use the lenses for after the trip to get full benefit of anything that you may buy. You can look at my African photos if you wish to get some idea of what lens is good for what.
http://www.flickr.com/photos/8364607...7630565377456/
http://www.flickr.com/photos/8364607...7601849785568/