Using last years Stats
http://www.uac.edu.au/documents/uai/2008_table_A3.pdf
Subject Mean SD Max. P99 P90 P75 P50 P25
Chemistry 31.6 9.2 50 46.5 42.4 38.7 33.1 25.6
Economics 30.7 10.4 50 46.9 42.6 38.9 32.8 24.1
Physics 30.4 9.5 50 46.4 42 37.8 31.6 23.9
http://www.uac.edu.au/documents/uai/2008_table_A5.pdf
Course Year Enrolment Percentage of students with scaled marks less than:
45 40 35 30 25 20 15
Chemistry 2008 10,154 97.1 80.8 58.4 38.0 23.3 12.9 5.8
Chemistry 2007 10,287 96.6 81.1 60.2 40.4 24.5 13.7 6.5
Economics 2008 5,410 96.4 80.5 58.4 40.4 27.4 17.0 10.0
Economics 2007 5,678 96.8 81.2 58.2 38.0 24.2 14.5 8.0
Physics 2008 9,029 97.3 83.6 63.2 44.1 28.0 16.0 7.8
These show why there isn't a straight answer. Based on the average it was Chem > Eco > Bio
Looking at the top end it was Eco > Chem > Physics
Looking at lower end it was Phys > Eco > Chem
Based on the top end and number in each course achieving the higher marks it was Physics > Chem > Eco
So they around the same and the stats can be used to argue whichever way you like. If you are in the top end of any of them you will receive a similar amount of boost