He was assessed by three specialists at Christchurch hospital, who cleared him for a trip to Antarctica.
He was eating in an Italian restaurant in Merivale, where press secretary Kevin Taylor said he fell to the ground in full view of other patrons.
The reason for his collapse was unknown.
“Sometimes faints just happen,” Mr Taylor told APNZ.
He is heading to Antarctica where he will learn about the Antarctic Research Institute’s work, before heading to a research spot in the Dry Valleys.
Felicity Plummer, restaurant manager, said Mr Key was sweating during his meal and did not look well.
He was talking to other patrons but it was clear he was struggling.
“I pointed out to my colleague I didn’t think he would make it away from the table.
“Within a second he had either gone into a faint or collapsed,” she said.
“We were concerned for him as you would be for anybody in that position.”
Despite earlier suggestions, it is not thought jet lag was the cause of the collapse.