Detectives, who have launched a hunt for a gang of five men, fear the 18-year-old victim has suffered a brain bleed.
Both he and his friend were set upon as they walked across a bridge from Mawson Street to Cabramatta Avenue at Miller about 8.20pm (AEDT) on Wednesday.
After a confrontation, the 18-year-old was sent plunging three metres into the creek below.
Police said the water broke his fall but he sustained his serious head injuries after climbing up a bank to fight back.
"It definitely was a vicious assault," Acting Superintendent Steve Corry told reporters on Thursday.
"The argument started between himself, a friend and five others. The young man was then pushed over the bridge."
After climbing back up the hill, he was punched and kicked by the group, Supt Corry said.
He was taken to a nearby house and the to hospital, where he was put in an induced coma.
"At this stage we are not sure of the motive," Supt Corry said.
"It appears to be a random attack."
Police said they had not established whether the victim had been taken to safety by his friend, who suffered less serious injuries in the attack and did not require treatment, or by someone else.
Officers from Green Valley local area command are attempting to find five men, described as being of Pacific-Islander appearance and aged in their early 20s.
Anyone with information is urged to call Crime Stoppers on 1800 333 000.
ninemsn.com.au 11 Nov 2010
So no one labels this as a possible racist attack, as racism does not happen in ethnic communities against white people?
What a joke!