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He also says the death of his father, when Fekitoa was a teenager, spurred him into providing for his family through rugby.
The 26-year-old Fekitoa, who played 24 tests up to 2017, is about to join English club Wasps after playing for Toulon.
Fekitoa, one of 13 siblings who grew up on the Tongan island of Ha'apai, told Britain's Mail on Sunday: "I left New Zealand to support my family.
"I don't just support my mum. I support my younger brothers and sisters, my nieces, my nephews.
"I've got close to 30 nieces and nephews around the world. Being able to support them is the best feeling ever."
Fekitoa said his father, and a brother and sister, had all died by the time he was 14.
"Everything crashed — Dad was a carpenter and he was the only person who provided for us," he said.
"Mum struggled for a while and that's when I realised no one was going to help us. That's when I put evrything on rugby."
The budding rugby star, who came to New Zealand on a scholarship aged 16, slept with his brothers and cousins in a hut, and lived by hunting and fishing.
"People talk about the struggles but life was great," he said.
"We didn't have much but we didn't need much. There were no TVs, no phones, no video games and that was the best part of it. We had the beach and the outdoors."
Fekitoa said: "Watching the All Blacks is always hard because I know what I can do. It's difficult but, at the same time, I feel proud watching the guys.
"I didn't just decide to leave. I thought about it for months and months and I believe I made the right decision. Whatever decision you make, you've got to back it 100 per cent and go with it.
"A lot more people are moving over now but in New Zealand there is always someone coming through who can fill in.
"The game is changing. It's a business. You don't play for ever and people are starting to get that now."
Fekitoa told the Mail that "people have started talking up Ireland but I still back the boys back home."