According to the Breast Cancer Foundation, 8 per cent of breast cancer cases occur in women under the age of 40.
"I can't even imagine what it was like for her, but you never expect it to be that close to home."
He said his sister had since fully recovered.
In New Zealand, breast cancer is the most common cancer for women and the third most common cancer overall.
One in nine New Zealand women gets breast cancer in their lifetime and it accounts for more than 600 deaths every year.
A Breast Cancer Foundation NZ spokesperson said on average 50 women in the Te Whatu Ora Whanganui area were diagnosed with breast cancer every year, and around 12 died from it, according to the most recent Ministry of Health statistics.
Inspired by various bikepacking books, McNaught said he wanted to get off the road as much as possible.
"Looking through those gets me super excited because there are some beautiful places to ride in New Zealand and most of the routes are designed to get you away from traffic."
McNaught will be gone for two months and around travel about 6000km.
He said he was circumnavigating the country as opposed to just going top to bottom, which is 3000km.
Starting from his parents' house in Feilding, McNaught plans to bike the North Island in December and make it home for Christmas, then do the South Island in January.
He said he and a friend attempted to bike the North Island in 2010 during the university summer break.
"We were super naive, didn't own road bikes and just did a couple of training rides on exercycles before borrowing bikes, bussing to the top of the North Island and just started riding."
He said they finished 900km before he tore something in his knee.
"After riding on that for a few days and in a lot of pain we just caught the bus home.
"It was still an amazing adventure, but we were so unprepared and every day hurt."
He said now was the chance to do it properly.
This time he made extensive preparations, biking up to 300km in September and then ramping it up to 800km in October.
"Now I'm trying to hit between 1500 to 2000km in November."
As part of his journey, McNaught is also on the hunt for the country's best custard square.
"Every Friday for about the last two years [Whanganui cafe] Honest Kitchen has put one of their custard squares aside for me because they kept selling out, but I just love custard squares.
"There's probably going to be some pretty rough ones out there, but the plan is to find the creamiest and best custard square."
He said he was also raising money for mental health awareness due to the prevalence of mental health issues in New Zealand.
"As a pharmacist, I see how many scripts are for anti-depressants and especially how many are for young people."
StatsNZ data show a significant increase in the proportion of people with poor mental wellbeing, from 22 per cent in 2018 to 28 per cent in 2021.
McNaught is setting up a Givealittle page raising money for the Breast Cancer Foundation NZ and the Sir John Kirwan Foundation.
