AusBonsai

I have a small euc. which I collected two years ago. As it was dug up from among soon to be developed scrubby bush with a variety of different trees on it I don't know what kind of euc. it is. Lately I have noticed that some of the leaves are getting a purple-ish discolouration and dry out and fall off after a while. Another euc. of a different kind which was dug from the same place at the same time is doing fine, and they have had the same watering and fertilizer regimes . Can anyone identify what the tree with the discoloured leaves is suffering from? Is it a disease or fungus? The newer leaves do not seem to be affected at this time.

I don't recognize the signs right off so these are only possibilities.
Nutrient deficiency is possible with the inter vein discoloration but some eucs are susceptible to Myrtle rust so have a look at Myrtle rust symptoms and see if it looks similar. The dead spots look right but Myrtle rust shows yellow fruiting bodies which I can't see in the photos.

Fortunately I have not seen rust down here so only working on pics for ID. Maybe someone further north will have a better idea.

The yellow pustules that Shibui describe usually tend to show up a fair bit later in the formation of the disease though, after it gets hold. But its not a given. The first signs are usually the purplish discoloration and decayed looking appearance on new growth as it slowly attacks the host.
Sometimes you don't see the yellow pustules until quite a while.

The underside doesn't look quite right for myrtle rust though. With myrtle rust it often attacks both top and bottom side of the leaf equally and also the stems, so it might just be rust, not really sure.

If it is myrtle rust, as John Mcclane from Die Hard said: Welcome to the party, Pal!
That is the part of your picture that doesn't look good.