Potted Christmas Tree Going Brown

Hello, gardening gurus, I have a 1.5 m potted Christmas tree which I keep under my front veranda. It does not get direct rain, so I water it weekly and it does not get direct sunlight, but gets strong filtered light and usually gets daytime temperature in Brisbane between 20-35c. Although the plant has been doing well in the past 18 months, it has recently started going brown along the central trunk and partway at the branches, all of the tips and new growth is healthy. Can you suggest what is causing the brown/discoloration?

By Therese