Eucalyptus Tree Help Please !

Hello !,Thank you for accepting my membership,and hope you can help !
I've recently moved into a new home, with (what I believe is a Eucalyptus tree in the garden, approx 12 - 15 feet tall, Mature leaves approx 6 inches in length.
Recently, the leaves have started turning very brown, but are not dropping at all, I've tried watering it, but there's a steady increase in the ratio of brown to green leaves.
I don't know if its lack of water, or fungus/pest but I'd like to save it if I can.
Any thoughts would be most appreciated, also should I remove the dead leaves, or leave them? should I cut the branches off with the dead leaves?
I've added a couple of pics, 1 showing the live leaves, the other the brown leaves.
Thank you for your help !

 

Can you show a picture of the whole tree.
They do tend to have the smaller lower branches die off as a young tree starts to mature.
They are still juvenile leaves in the picture.
I doubt it's a watering problem as they are pretty drought tolerant.
Is it making new growth higher up, they do get very big quite fast.

 

Hi Pete,
Thank you for your help, I'll take a photo soon .... if it stops raining !

 

Sorry it's taken so long to post new pics ..... they're not the best, access is tight, but I hope these help solve my problem .... thank you ! (1st picture is from above)

 

If it's the lower branches i wouldn't worry, just cut them off flush with the stem.
It looks pretty healthy otherwise.