He was regarded as a regular Test contender before suffering a concussion in early 2016 which sidelined him for more than a year.
Ngatai said he had considered retirement before making a return in May last year, and admitted his decision to head north was influenced by the head knock.
He said he wanted to secure his financial future now, rather than try to push into All Blacks contention ahead of the 2019 Rugby World Cup in Japan.
"That's another reason why we looked overseas as well, you never know when your career's going to finish," Ngatai told reporters.
"I guess a lot of it comes down to money and then opportunity - there's a lot of young players coming through in New Zealand.
"Places in the All Blacks team are a lot harder."
A standout age-group player, Ngatai played his early Super Rugby for the Hurricanes and has represented Wellington and Taranaki at NPC level.
He'll follow compatriots Lima Sopoaga and Brad Shields in the northern hemisphere, and join former All Black Rudi Wulf in Lyon.
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