A 29-year-old man has been bludgeoned to death in broad daylight at Sydney Airport in a brawl that police believe involved up to 15 outlaw bikies from possibly two gangs.

The man was treated at the scene after being hit with a portable metal bollard but was pronounced dead at Prince of Wales Hospital. The bollard, among a number of other items, has been taken for forensic examination. Five crime scenes have been established - one at the hospital and several at the airport.

Police said a group of men arrived on a flight to Sydney about 1.30pm on Sunday and were unexpectedly met by another group of men, possibly a rival outlaw bikie group.

"A fight ensued [and] moved through various parts of the terminal," Detective Inspector Peter Williams said. He said it "appeared that a certain degree of planning went into it".

Police will review CCTV footage and have begun speaking to between 30 and 50 witnesses, many of whom Detective Inspector Williams described as "traumatised".

Police have four men in custody who are assisting them with inquiries.

Sydney Airport said the matter was now with police so they were unable to comment.

