That day led her to realise she wanted to become a nurse.
The now 19-year-old is in her first year of a bachelor of nursing degree at Otago Polytechnic and is chasing the career to help others who may have dealt with similar circumstances, as she did.
"Seeing mum sick, you see the nurses and the doctors working behind the scenes and just the way that they've looked after her.
"When I [used to] go in and visit her, I used to be terrified, like so scared. It's such a scary place in there.
Fewer than 10 people in New Zealand had the form of cancer that her mother, Kirsten, had and there was no cure, she said.
"She's got her ups and downs. She's definitely a huge battler, very resilient.
"She's taught me a lot and her team that look after her are really amazing as well, and good support."
The initial phase of her mother’s diagnosis in 2018 was "challenging", she said.
"I found it really hard to tell my friends and kind of have a connection with them and none of them really understood what it meant.
"[Mum] hasn't been able to get better because there's no cure, but then she hasn't got worse either yet."
Since then, Ms Arnold has been heavily involved with CanTeen Aotearoa and was now part of their leadership programme.
They offered her support and made understanding her situation easy, as they gave her people to talk with who understood her situation.
"There's no ways to describe how much of an impact that is. It makes you feel less alone."
The support of CanTeen Aotearoa gave her the confidence to speak about her mother’s diagnosis with friends and whānau.
The group provides a range of support services for people aged 12-24 affected by cancer.