Dying Eucalyptus- please help

Hi, I am hoping someone can advise the best way to save our eucalyptus tree. I really am clueless with gardens.
Nov/Dec leaves started turning brown on one side, this has not happened through any other winters. It has now turned the grass yellow and also changed the colour of the rockery.
The guy who cuts are grass put “round up” (presume weed killer) on the outside of our fence but presume as we live on a hill it has ran onto the rockery and tree. Could he have killed it?
A saving grace is that half of the tree at the top still has green leaves. Please see image I have uploaded
Please can anyone advise best course of action.
Thank you.
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@steephill, would the tree recover, or is it dying?
I was going to suggest cutting it right down to see if it reshoots?