The documentary-style comedy, which follows three teenage loners as they try to win popularity by throwing an out-of-control house party, made almost $1.3 million in ticket sales.
Mark Wahlberg's Contraband, which took the number one spot last week, drops down to second place, followed by Reese Witherspoon, Tom Hardy and Chris Pine's This Means War.
The Vow makes number four in the list after taking $894,606 in ticket sales, while new entry The Devil Inside follows in fifth spot having made $736,658 profit.
French silent film The Artist jumps up from number 10 to seven in the list, possibly boosted in sales because of its multiple wins at last month's Academy Awards. Martin Scorsese's Hugo, also an Oscar winner, re-enters the list at number eight.
The Australian box office top ten in full below (figures in Australian dollars):
1. (-) Project X - $1,279,691
2. (1) Contraband - $1,162,537
3. (2) This Means War - $1,003,646
4. (3) The Vow - $894,606
5. (-) The Devil Inside - $736,658
6. (4) Safe House - $708,344
7. (10) The Artist - $559,589
8. (-) Hugo - $412,630
9. (6) My Week with Marilyn - $330,901
10. (5) The Grey - $310,380
Watch the Digital Spy world exclusive clip for Project X below: