The latest cases include a seven-month-old, a three-month-old and a 19-year-old.
The two infants are currently in a hospital isolation ward.
The Outbreak Response Team is conducting measures such as isolation, quarantine and vaccination of contacts and at risk communities.
During the weekend, health authorities were notified of a suspected case of measles within the Tongan rugby team that had arrived to play in a regional tournament.
The suspected case has been isolated and the team placed under quarantine with their scheduled games cancelled pending test results.
The Ministry of Health said vaccination stocks were not unlimited and should be used for the most vulnerable groups, including any child who had not yet received the two doses of the measles vaccine.
Anyone who lives in Serua/Namosi, over six months of age, is urgently advised to receive a dose of the measles vaccine regardless of prior vaccination status.
People visiting the area have also been urged to get vaccinated at least two weeks before travel.