Vaha Mafi has lost more than 70kg of her body's weight in 15 months.

The Auckland woman weighed 138.7kg when she decided on Waitangi Day - February 6 - last year that it was time for radical changes in her diet and physical activity.

Now, the 29-year-old weighs 68kg. That marks a loss of 70.7kg, more than half of her starting weight.

She was concerned about the potential harm to her health from being obese, but it was visiting her brother, a Wellington public health worker, which triggered the dramatic turn-around in Mafi's life.

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"I was actually struggling with the complications of my weight problem but I never wanted to admit it. Instead I was pretending that I did not care and, even worse, forcing myself to pretend that I was happy and satisfied about it.

"I'm so thankful to my brother, Siaosi Mafi. Through his own journey, he was able to share his experience and knowledge with me."

"I came back Monday evening, from my brother, and started on Tuesday [February 7 last year]. I started going to the gym on Tuesday evening. There was a 12-week challenge at the gym."

She nearly quit during her first week on the challenge, finding that she was last to finish the cardiovascular exercises. "I said to myself, 'Don't give up, keep going'."

By six weeks, she started to feel her fitness increasing and she was enjoying losing weight. She even started doing some exercise in addition to the challenge, going for a jog each morning before work and taking some boxing classes.

By the end of the challenge she had lost 30kg. After the challenge, "I kept going by myself".

Mafi said changing her diet had been difficult.

"The hardest thing in that journey was my food. I like fizzy drink and takeaways."

But she had made the changes she considered necessary, eating a lot more vegetables and drinking water.

Mafi said her brother's input was critical.

"It was not so much about nutritional tips or exercise but rather about emotional connections to food and the influences of cultural factors.

"I had a positive mindset to begin with, and told myself that I will do it no matter what happens along the journey.

"It was tough but I knew I had to make some sacrifices, especially letting go of all the foods/drinks I was obsessed with and learning to be more active.

"The reality is there is no finish line in trying to live a healthier lifestyle … No doubt there will be hiccups along the way but that's no problem, you pick yourself up and keep moving [forward] …"