The Aussie rapper made an appearance at the Culture Kings store in Melbourne, and part of the store was shut down Sunday ... so he and his crew could do some shopping. Things took a turn, though, when he and his entourage decided to play hoops on the store's indoor court.

It was close quarters, for sure, and at one point The Kid jumped pretty damn high for the ball, only to trip over one of his pals ... and come crashing down right on his tailbone! Immediately, you could see he was in pain.

While a lot of fans thought he'd have to cancel his concert scheduled for that night, TKL stuck to that old adage -- the show MUST go on -- and got onstage that night ... with The Wiggles no less!!!

He certainly didn't look any worse for wear as he went HAM performing his childhood heroes' classics like "Big Red Car" and "Fruit Salad."

The Kid had already postponed the Melbourne concert the week before, so he knew he had to put on a great show, regardless of how sore his back must have been.