"I was looking at taking some leave over Christmas," Dr Mills said. "They said: 'Look, we really need someone to come now'."
This week a report by the Africa Governance Initiative found Ebola was spreading nine times faster in rural Sierra Leone than it had been two months ago.
The outbreak, focused in West Africa, is the largest on record with more than 5000 dead.
Dr Mills' role is unlikely to put her in direct contact with Ebola patients and will centre around co-ordinating the medical response to the outbreak.
She will be supporting teams in the field through training and clinical advice, liaising with Sierra Leone's health ministry and co-ordinating MSF's response with other humanitarian organisations.
When asked about the potential of contracting Ebola, Dr Mills said there was always an element of danger but she was confident with the protocols in place.
"There's a calculated risk here but at the same time I know what I need to do to protect myself for it," she said. She will go through the 21-day quarantine required before coming back to New Zealand.
Dr Mills spent four years working for the Dutch section of MSF in Amsterdam from 2004 as the medical director. She has also worked in Ivory Coast, South Sudan, Northern Uganda, Sri Lanka, Rwanda, Papua New Guinea and Eastern Zaire.
The New Zealand government needed to put more resources into combating Ebola, Dr Mills said.
"If we want to be a big [international] player then we need to get on board," she said.
Dr Mills' Ebola-fighting efforts come after NDHB midwife and women's health nurse Donna Collins worked for four weeks at the Kenema Ebola Treatment Centre in Sierra Leone from August.