Of the nine, five are people who are travellers to the territory.
Epidemiologist Dr Aifili John Tufa announced the outbreak during a live broadcast fundraiser to help neighbouring Samoa's fight against the disease.
Dr Tufa later told KHJ News they believe there are other cases that have not presented at the hospital.
He said this means they couldn't track these people, who may have exposed others, spreading the disease further, and the Department of Health does not have any way to track them, "and some of them, we have identified, they were sick with the measles here, they got better but did not present at the hospital. So that way we cannot track the numbers. So that's the concern and we are sure these are not isolated events."
The outbreak began in Samoa two months ago and, as of today, 65 people have died, and more than 4,000 people have been infected.