How to save this cumquat tree?

Melbourne's extreme heat and lack of frequent watering has left this tree at my parents' house almost dead.

Any ideas on how we can save it? I water it as much as I can when I visit but my parents aren't very diligent with it.

Melbourne's extreme heat and lack of frequent watering has left this tree at my parents' house almost dead.

Any ideas on how we can save it? I water it as much as I can when I visit but my parents aren't very diligent with it.