The group - who won the fourth season of Australia's Got Talent - fend off competition from Maroon 5's 'One More Night', which jumps four to two.
Elsewhere in the top ten, B.o.B and Taylor Swift leap 11 to six with 'Both Of Us', and Florence + the Machine bound 14 to eight with 'Spectrum (Say My Name)'.
Further down the chart, Calvin Harris debuts at 11 with 'We'll Be Coming Back', Ed Sheeran's 'Drunk' vaults 37 to 12, and Johnny Ruffo's 'On Top' shifts 24 to 15.
Timomatic rise 29 to 20 with 'Can You Feel It', The Veronicas start at 29 with 'Lolita', and 3OH!3 begin at 32 with 'You're Going To Love This'.
Outside the top 40, Ricki-Lee's 'Crazy' is new at 46, and Cher Lloyd debuts at 49 with 'Want U Back'.
The Top 10 singles in full (click where possible for our reviews):
1. (2) Justice Crew: 'Boom Boom'
2. (4) Maroon 5: 'One More Night'
3. (1) fun. 'Some Nights'
4. (3) Rudimental ft. John Newman: 'Feel The Love'
5. (11) B.o.B ft. Taylor Swift: 'Both Of Us'
6. (6) Owl City ft. Carly Rae Jepsen: 'Good Time'
7. (5) Pink: 'Blow Me (One Last Kiss)'
8. (14) Florence + the Machine: 'Spectrum (Say My Name)'
9. (7) Katy Perry: 'Wide Awake'
10. (8) Justin Bieber ft. Big Sean: 'As Long As You Love Me'
Watch Justice Crew's 'Boom Boom' music video below: