Your UAI is more dependent on your performance in the HSC examinations than internal assessments. Whilst it is also dependent on internal ranks, there is NO set numerical value for a particular rank. So hence, there is NO GUARANTEE. Internal assessment tasks are not as significant as the rank for the final exam.
Without going to the complicated issues, for example if you are doing really well in your internal assessments but you do badly in the HSC examinations, then that assessment mark (hence final mark) will be dragged downhill. On the other hand if you are doing badly in all your internal assessments but you do really well in the HSC examinations then your assessment mark (hence final mark) will be pulled uphill.
So even if you get top 5 across all courses, if you perform poorly in the HSC examinations then your overall score would be low. However, if you perform well in the HSC examinations then your overall score would be high.
Therefore, your performance in the external HSC examinations is probably the largest contributor to your UAI.