At least one of those hospitalised was a baby, one developed pneumonia and two of those requiring medical intervention were aged under nine according to Waikato District Health Board medical officer of health Dr Richard Wall.
"Twelve people have been hospitalised which is higher than we would expect. The rate is normally one in 10."
Twenty-one of the victims were aged between 10 and 19, while 22 of the cases related to children aged four or younger. No cases have been fatal.
Wall said dehydration was often a common complication of measles and why some victims ended up in hospital.
The Waikato had the highest number of cases but other regions were also affected by the outbreak.
The warning was aimed at age groups where vaccination rates against measles were not as high as they are now.
"Where the spread seems to be happening is in the teens through to the early 20s which is probably the age group that will be there (at the kapa haka event)," Wall said.
Teenagers who caught the virus often spread it to other family members while babies under 15 months, the age which the measles, mumps and rubella vaccination is recommended, were most vulnerable.
Wall said it was a Waikato resident who first contracted measles in the latest outbreak and it spread from there.
He said though it wasn't the highest outbreak on record, 56 confirmed cases was a lot.
"Obviously the wish is for none. But we've hit more in other outbreaks."
Cases were reported at schools in Morrinsville and Hamilton and on a flight from Rarotonga to Auckland on April 24.
One of the victims included a Waikato Hospital worker who had been vaccinated against measles but still contracted it.
Medical Officer of Health Dr Felicity Dumble said for those who receive two doses, the measles vaccine provided protection in more than 95 per cent of cases.
"This was a rare case when the vaccine has not provided sufficient protection."
Because of the incident all Waikato DHB staff were asked to check their MMR vaccination status, especially those who worked in clinical areas and those employed before July 2015.
At that time the DHB put in place a vaccination policy that all new staff must be vaccinated against measles, mumps and rubella, hepatitis B, varicella (chickenpox), tetanus, diptheria and pertussis (whooping cough).
An MMR immunisation is given twice at least one month apart for immunity.
Dumble said measles can't be treated once you get it.
Facts about measles
The first symptoms are fever, and one or more of a runny nose, cough and sore red eyes
After a few days a red blotchy rash comes on and lasts up to one week. The rash usually starts on the face and spreads to the rest of the body.
Measles is infectious before the rash appears.
Common complications include dehydration, pneumonia, diarrhoea, and ear infections. A more serious complication is encephalitis which can lead to brain damage.
Notified cases must stay in quarantine for two weeks as well as anyone who associates with that person who does not have proof of immunisation.
Anyone born before 1969 is likely to be immune to the disease without having had the vaccine.
