why do you need to convert?For application purposes? What qualification do you hold? It is not possible as far as I know to convert ATAR to CGPA as the way you get the same ATAR is not the same each year.

Collect all the cgpa results of all student doing the same course as you, eg all the stpm and matrik results, rank all the students, then see where your cgpa is in the list, and that is your atar rank.........

ie if your cgpa is 3.7, and there are 15% of students with higher cgpa than you, your atar is 85.........

hum that's weird asking for ATAR from somebody trying to do a credit transfer ^^" Did you state that in your application form? ^^"
usually most Aussie unis asked for Weight Average Marks rather for cases like that as far as I know. You should email Macquarie to clarify the matter.