This really depends on your school
some schools set really hard assessment tasks through the year and therefore their internals are scaled up.
If you are on average then your internal mark will be scaled to your schools average in the external exams. If the average is 60, than your school mark will be 60 too, it's not quite that simplistic but you get the idea.
What you need to do is work your arse off for externals and hope the rest of your grade is doing the same thing... otherwise they might ruin your internal mark for you.
good luck