Matt Damon Struggles on Surfboard While Down Under

Matt Damon got dumped in Australia ... by the waves, not his gorgeous wife.

Matt hit up Byron Bay for a little surfing session and he got off to a pretty rough start. You can see the action star show off some beginner moves -- falling over and over and over, and ... ya get the point. Jason Bourne's stunt double was not on the beach.

Still, gotta give him credit for getting back up and giving it another go. Check out the clip ... there's no quit in Matt.