| Probably not so much sun as dry atmosphere. They come from the Three Kings Islands off the tip end of Northland and tend to pick up the tail ends of cyclones dropping rain in hefty amounts. Yes, we do have dry spells, but not as sustained as yours. They grown in full sun in the Wellington Botanical Gardens - and with a lot of the windiness for which Wellington is notorious. About 1km from the harbour and definitely gets salt spray. No harm. It will tolerate a semi-shaded position. However, you might also want to mulch fairly deeply after a good spell of autumn rain and see if that brings an improvement. If you can make a micro-climate that will raise the humidity level around the plant, too. When it's not raining ;-) the humidity is reasonably high...(70+) eg for this evening it's around 88%. | 
