NOFX Singer Fat Mike -- Takes Down Fan ... One Punch, One Kick (VIDEO)

NOFX lead singer Fat Mike blasted a concertgoer in the face TWICE Wednesday in Australia ... but the guy isn't planning to sue -- in fact, the young fan is taking the blame.

The famous punk band was playing a gig at the Enmore Theatre in Sydney when an overzealous concertgoer wearing a bucket hat jumped on stage and put his arm around Mike's neck.

The singer had told the crowd earlier in the show he was suffering from neck pain ... so Mike reacted with a spinning forearm to the guy's face -- followed by a punt kick to the head while he was down.

The young fan -- named Alex -- tweeted Mike after the show apologizing ... saying he had been drinking and forgot the singer's neck was hurting.