what if u rank first internally but completely screw up your externals? are u still safe or not
Depends what you mean by safe, and what you mean by screwed up.
Could be a scenario where the highest external mark is 60, and you've screwed up and gotten a 54, then your final HSC mark is gonna be (60+54)/2, being 57.
Or highest external is 100, you've screwed up and gotten a 54 again, then your final HSC mark is gonna be 77.
Or, highest external is 100, you've "screwed up" and gotten an 86, then your final HSC mark will be 93.
Can't tell you with certainty if its safe or not mate.