randwick girls is like in 300s. 98+ is still very possible if you focus more on your chem and phys (since that you are set for band 6 in esl and maths).
minimal effect. because the marks are extremely tight (depends how tight. ~1%), if you ace your external, you external mark will be the internal mark of the person who got 1st. and then calculate yours. so i think you wont be affected much.