the national undergrad one is like the scientia, UAI 99.9+ or equivalent, you keep it for the duration of the program but have to maintain a certain WAM (70+ i think). it's a bit more money ($12 500 p.a) but that extra moolah will get soaked up by your residential college fees.
national merit scholarships are UAI 99.5+, WAM 70+, $6500 p.a.
regional ones are for people from certain regions in Australia, based on UAI or equivalent and community work...
there are more but i can't remember them all