Hi Emily,
I have a Eucalypt Leucoxolon which is about 4 meters tall. It has also had small brown spots on the leaves, which then die and fall off leaving the lower half of the trees branches leafless. On two sections the bark has cracked also and I am wondering if this has anything to do with the spots and die-back. The top half of the tree appears normal and is flowering though not many. My first thought was 'rust', or some type of fungus, though from looking at some sights that indicated that E. rust is not yet in this country, and the pics show the brown spots with a yellow halo - mine did not have the yellow halo, so as to whats affecting mine remains to be identified. Whilst looking up info for you, I found the following sites to be helpful, so hope they may be of some help to you.

Australian Plants online
http://farrer.riv.csu.edu.au/ASGAP/APOL5/mar97-4.html

Association of Societies for Growing Australian Plants (ASGAP)
Kangaroo Paws: Pests and Diseases
http://farrer.riv.csu.edu.au/ASGAP/paws1.html

Gardenweb : Australian Native Plants
http://www.au.gardenweb.com/forums/oznative/

Electronic Mailbox
http://farrer.csu.edu.au/ASGAP/APOL35/sep04-8.html

There is also the following article which I found on the above website.
Rust on gum leaves
I have a 3 metre tall Corymbia "Summer Red" grafted gum that is my pride and joy. It's growing in full sun, clay soil, is staked, and flowered profusely last summer. Now it has begun to get a red rusty rash-like appearance on its leaves. I think I should cut off the lower branches, as they touch the ground, but should I then spray the tree with a fungicide?
M H from New South Wales

It's almost impossible to diagnose these sorts of things from a distance. The foliage discolouration could result from a range of causes, including fungal (rust) diseases, soil nutrients or lack of water. It could even just be a response to cold weather. You really need to take a sample to a reputable nursery in your area or, better still, a local NSW Agriculture office if there is one near you.
If it is a rust, then spraying with a fungicide may help. You can see some photos of a eucalyptus rust in this document - this may help you decide if it is a rust problem.
If it's not a fungus then probably there's not a lot you can do at this stage other than making sure it doesn't dry out. If it doesn't recover with new growth once the weather warms up, you should definitely get some local expert advice.

Good Luck!
ladyecheveria

yeah, the bad one of mine is too an E. leucoxolon, and there seems to be no identical match to any pest or disease in the pest and disease book. i will have to remove the near dead tree and i will start spraying a fungicide on the others, hopefully that will control it, this is what it looks like:
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yeah, the bad one of mine is too an E. leucoxolon, and there seems to be no identical match to any pest or disease in the pest and disease book. i will have to remove the near dead tree and i will start spraying a fungicide on the others, hopefully that will control it, this is what it looks like:
{{gwi:2116336}}
Eucalypt leucoxylon
{{gwi:2116337}}
Eucalyptus 'summer red'
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unknown Eucalypt, if any one knows id...
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Eucalyptus ficifolia

sorry, first picture post, not meant to be there