By undertaking the Pink Spring Spin, the two are also hoping to raise money for the New Zealand Breast Cancer Foundation - a disease both had in their families.
Nine years ago Ms Mackie's mother was diagnosed with breast cancer. Thankfully, Ms Mackie said after receiving treatment she survived, and was now "thriving".
"I just really feel it's important to find a cure, to at least give other people the chance to have what my mum's got now," she said.
As the pair had called New Zealand home for the past eight years, Ms Mackie said "we just really wanted to give something back to New Zealand and raise some money for a cause that seems to be affecting a lot of women here too."
The pair - who are both physiotherapists and involved in triathalons - had been training intensely for their 2100km journey.
For Ms Mackie, now based in Auckland, this meant riding up to 20 hours a week, sometimes training twice a day, on top of swimming and running. She would then spend up six hours on the bike on the weekends.
Mrs Gillies, who had never spent more than two hours on a bike before, had been training twice a day for the past three months, with up to 5-hour rides on the bike on the weekend.
Preparing the logistics of the ride had presented its own challenges - having to find new sponsors for campervans on the journey, and having their kit caught up in Customs, with a $400 GST fine.
However with the support of their respective employers, families, training partners, and sponsors, months of work paid off when the pair were able to get on their bikes yesterday.
They will accompanied on their journey by another eight people, sharing the ride with two campervans, a van, and motorbike.
For Ms Mackie knowing there was 2100km to ride was very daunting.
"Its 42 hours," she said, "the rides a full time job for the next week sitting on my bum and pedalling, that's a bit scary."
Although it was a "huge undertaking", Mrs Gillies said they would be riding in manageable bites of three hours at a time.
"If I think about the whole ride I'd get overwhelmed but I just think I have to ride for 3 hours, and then I have a 3 hour break, and that's just how I'm going to approach it."
-If you would like to help the pair on their journey, visit https://givealittle.co.nz/fundraiser/spingspin