After seeing this post, someone emailed me to say they'd had a look at the tree and it was fine.
I thought I'd share my reply in the interest of helping others recognize stressed trees. It can be quite subtle until you get your eye in. Then it stands out like a sore thumb and you'll start seeing them everywhere!
Hi [name removed],
The tree is definitely showing signs of stress as evidenced by the following.
The foliage is:
a) Thinning as the tree sheds leaves to try and reduce its rate of transpiration
b) Limp due to a lack of turgidity (ie water deficit in the leaf cells)
c) "Dull" - ie lacking the usual dark green lustre of this species
The easiest way to see this is to simply compare the tree with the one next to it (not visible in the photo) which is not yet showing these signs.
The time to act with the tree is now - ie before it gets worse. Eucalypts can reach a "tipping point" quite quickly from which no amount of TLC brings them back. It would be a shame to let that happen in this case as it's such a fine specimen.