I would dig it up ASAP and pot it up into a well draining mix. There is a chance to save it by doing so because you control everything from water, sun and fertilizer.

Looking at the tag, that is a NZL.

Looks like it may have been planted too deep. The shallow-rooted plants prefer to have the root flair exposed and slightly above the soil line, although it is hard to tell from the angle of your picture whether it was planted as a mound (it may be).

Also with it in the ground that long, I expect many in-ground planters here would have recommended removal of the stake or at least have looser ties on it so that the trunk can build up some strength while swinging in any breezes. With my FWG NZL, I cut the original white tie strips and am using the green stretchy tape to keep it loosely upright in its pot near the stake. Finally it looks like it needed some food, which you said you gave it.