Mrs Spenceley (50), a lecturer in midwifery at the Otago Polytechnic, had been experiencing back problems and was told by therapeutic masseur Rowan Ellis to lose weight.
"I'd tried to lose weight from the age of 7 and been on yo-yo diets that didn't work," she said. "Rowan told me this diet would work."
Mrs Spenceley, who emigrated to New Zealand 17 years ago from England, weighed 83kg a year ago. Her weight now fluctuates between 53kg and 55kg.
"Kate has a lot of determination. I knew she could do it when she rode 80km on her first bike ride," Palmer said.
Mrs Spenceley, who lost her weight through a combination of exercise and diet, trains for about 20 hours each week and combines swimming, cycling and running.
Her diet is high in protein and low in fat and carbohydrates. It is a body-builder's diet.
On a typical day, she eats porridge and fruit (breakfast); protein shake and orange (morning tea); large salad and protein shake (lunch); protein shake (afternoon tea); fish or chicken, vegetables and fruit (dinner).
"I've never been so fit and healthy," Mrs Spenceley said.
"The diet is easy and I'm never hungry. If I don't eat I lose more weight.
"I spent years thinking about my weight and that is why I got so fat. I have made a lifestyle change and lost a third of my bodyweight. It is fun and the diet is easy."
Mrs Spenceley has the backing of her husband, Graham, and their two children