So why are MBS rebates for longer mental health consults lower than a normal level C?

The Department of Health and Aged Care has the answer
Medicare card

Given the deep damage mental illness can inflict on a patient’s life — on their well-being, their sense of identity, on their intimate relationships, on their work — even based on the brutal calculus of cost–benefit economics, you would assume funding for healing the mind would be a national priority.

Not quite life or death, at least not always, but something close.