no one can accurately predict the uai cutoffs - as you said they are merely an indicator of the number and quality of the applicants relative to the number of places available. but the cutoffs generally don't change too much from year to year, so try not to worry too much about it.
also UNSW has quite a generous alternative entry scheme for engineering called FEAS (
http://www.eng.unsw.edu.au/feas/index.htm). i know someone who was accepted into electrical engineering (cutoff 85) with a UAI of 75. this scheme takes into account your potential as an engineer by looking at subject marks individually and also interviewing you.
can i just say this too: are you interested in mining and petroleum engineering because you have glanced at the average salaries for both of these?
i am not trying to put you off either of these careers, but both of these types of engineering normally require a drastic change of lifestyle: for mining you will be living in the middle of nowhere in Western Australia probably, and with petroleum you could well end up on an oil rig in the Indian Ocean for months on end.
money is definitely not everything in a career. just think about where your interests and abilities truly lie, and i am sure that you will exceed in any field that you choose.
anyway good luck with the HSC and all the best for the future!