The law here is
clear and settled.
In Tasmania, a 15 year old is, as a matter of law, incapable of consenting.
Therefore, absent consent, you are looking at sexual assault, and, for certain acts, r**e.
This is true no matter what the child actually says,
and no matter how <ahem> personally sophisticated that individual child may be.
It's also true even if the other party is (in Tasmania) 17 or over.