When asked why she wanted to cut her hair, Sadie said it was "to help people".
Mikayla said her other sister Ella Rose thought it was a "really cool idea".
"You know you've done it for a good cause," Ella Rose said.
The girls' mother, Kiri Hall, said she was really proud of her daughters. They would be getting their hair cut on May 16, when 30cm to 35cm of each girl's hair would be made into a wig.
Ms Hall's friend died from breast cancer five weeks ago and they had decided they would do the chop on her friend's daughter's 16th birthday.
Another one of Ms Hall's "very best friends" was diagnosed with breast cancer in June and has had 16 weeks of chemotherapy.
Mikayla said she hoped their hair would go to women who needed it.
"Just anyone who is feeling insecure about their hair, if it can help them grow some confidence, that would be really cool."
Mikayla said she "can't wait" to get her "really curly" hair chopped off into a bob.
"It's been months and months of struggles to get the knots out and having to brush each other's hair," she said.
She encouraged anyone who had thought about growing their hair for cancer patients to go for it.
"Hair grows back, but once you've had cancer, you can't really get away from it. Just help people out, I don't know why you wouldn't."
By 4pm yesterday the girls had collected $1645.
They hoped to raise at least $2000 for the Breast Cancer Foundation.
Can you help?
* To donate, go to: givealittle.co.nz/fundraiser/carterhallgirls