Without ranges, it is hard to tell about tests (labs, countrys vary).
My guess is a lesion that large is a non-functioning pituitary lesion. Hate to say this but the eyesight likely won't come back.
As for cure... I hesitate to use that word and especially with a tumor so large and probably complex as your father's lesion. He will need a super experienced surgeon so he can maintain the eyesight he has, and also so that his hormones stay normal (I think they are normal).
Given his tumor has grown up into the optic area, the surgeon cannot remove all of it. He/she will only be able to debulk it. He will need radiation likely to kill off more of it, but if any is left, it can grow back and these tend to be fast growers (if that is the tumor type).
He needs a neuro endo at a pituitary center and a very skilled surgeon. Look into radiation types (they will push what they have, which may or may not be the best) and get copies of everything.
If it was a prolactinoma, prolactin levels would be high. And yes, medications could be used to debulk (normally) but his lesion is so large, I bet they would opt for surgery since he has lost vision.
I live in the US so I am not familiar with any centers in Australia. I always say to look at university hospitals an large hospitals. They are more likely to have them.
As for eating anything help... Wow I wish. His body is working against him big time and he needs treatment before things will really turn around.
As for the US, I would contact the NIH in Maryland as they do studies and may take on your father for a lower cost.