Neither the Australian Federal Police (AFP) nor the Ambulance Service of NSW would comment on media reports the man was a drug courier trying to smuggle heroin into Sydney.
Ambulance officers saved the man by administering Narcan, which is used to treat heroin overdoses, the reports on the Seven and Nine networks said.
The man, aged in his 30s, was placed in quarantine after being taken off a Vietnam Airlines flight which arrived at Sydney Airport from Hanoi via Ho Chi Minh city this morning.
NSW Health Communicable Diseases director Jeremy McAnulty said initial tests at St George Hospital showed it was very unlikely he had the deadly avian influenza.
NSW Health Minister John Hatzistergos said he did not want to comment on the matter as the AFP had become involved.
The AFP said that the man was a person of interest but would not comment on the drug courier allegations.