In the end, your assessment mark (internals) and HSC exam mark (externals) are worth 50% of your final HSC mark each. I wouldn't say one is more important than the other as you need to be performing to the best of your abilities in both your internal assessments and trial exams (which is manifest through high ranks) as well as achieving a good result in the HSC exam if you wish to receive a high ATAR.
As mentioned above, with the moderation process, the highest assessment mark is adjusted to equal the highest exam mark of any student in the school group.
Have a look at this video from NESA in order to gain a better understanding of the moderation process: