A man suffered life-threatening head injuries when he was assaulted in south Auckland early this morning.

The man in his 40s was taken to Middlemore Hospital in a critical condition after the incident on Moncrieff Ave, Clendon Park, shortly after midnight.

St John Ambulance communications team manager Norm Ngatai said it was unclear whether the man's head injuries were sustained in the assault or from his head hitting the ground.

Police were investigating.