Sorry, but we need more information in order to answer, so one, How long have you had the plant, two, what is your USDA zone (assuming you actually are in the USA), three, any evidence of infestation or infection on the plant, etc.

I think he is saying zone 9a Bamboo.

Ah, well that's been added since I responded, MG. I see your winter temps range bewteen -6.7 and 3.5 deg C, but I don't think its winter there currently, so temperature dropping too far wouldn't explain it.
You haven't said how long you've had the plant, but water might be the problem, and possibly the ph of your soil. They prefer soil to be slightly acidic, and they do not like to dry out completely, so soil that retains some moisture is preferred. If its in hot sun and you only planted recently, its possible its getting too dry and this is causing a problem, or the ph of your soil is on the alkaline side.

New Zealand Tea Trees don't like heat and high humidity combined. When we are crazy enough to try them here in the desert, they always die during the monsoon season, sometimes one branch at a time. Unless you see a number of them in your neighborhood, I would expect it to be a poor choice for your climate. Much easier on the west coast, from Los Angeles to Gold Beach.