I seem to remember reading that eucalyptus leaves change from oval to longer and thinner as the tree gets older, try scratching the soft bark of a thin twig , you are looking for healthy wood underneath, not dead dry brown.....

I believe your winters are mild, but Eucalypts should be pruned in spring, when growth is beginning anew, not in winter. The fact it was pruned in winter doesn't necessarily mean that's the cause of the problem though.

You haven't said which variety of Eucalyptus you have, nor attached a photograph, but discoloured leaves could indicate a phytophthera (fungal) infection at the root or base of the trunk. Also known as root, foot, collar or crown rot. Inspect the tree thoroughly, particularly at the base, but also to see if you can find anything that shouldn't be there on the trunk or branches, either insects or splits, cracks, seeping wounds, anything like that.