Ten cases of measles have been confirmed by the Ministry of Health.
One confirmed case in Auckland and nine are in Turangi and Taupo.
There is a further case under investigation in Turangi/Taupo.
Ministry of Health acting deputy director of public health Dr Harriette Carr said the Auckland person and three of the nine confirmed in Turangi/Taupo had attended the 2013 World Supremacy Battleground hip-hop competition in Sydney last month.
The remaining cases were family or known to those who had been at the festival.
No one has yet been hospitalised but all those currently infectious are being kept isolated at home.
Public health authorities were working to identify people who may have been in contact with those affected, Dr Carr said.
"Measles can be a serious disease and is easily spread to someone who is not immunised, or who has not been exposed to measles previously.''
Public health unit staff were following up with those who had been in contact with those infected, and emergency departments had been alerted to watch out for possible measles symptoms.
"It's very important if people have symptoms of measles that they seek medical advice. Symptoms can include fever, runny nose, and sore watery red eyes that last for several days before a red blotchy rash appears.
"Because measles can be easily spread, it's also important that people contact the Healthline 0800 number or ring their doctor first so that their symptoms can be initially assessed without risk of infecting others in a GP waiting room or hospital emergency department,'' Dr Carr said.
The cases were a reminder that the most effective protection against measles was immunisation.
A publicly-funded vaccination was available via GPs. People should also make sure their routine vaccinations are up to date before travelling overseas.
What to watch out for:
* Measles symptoms include fever, a runny nose, sore and watery 'pink' eyes and sometimes small white spots on the back inner cheek.
* The illness usually starts 10 to 12 days after exposure.
* A rash usually starts on the third to seventh day of the illness on the face, or behind the ears, before moving over the head and down the body.
* The virus can cause pneumonia, ear infections, and inflammation of the brain.
* Anyone who suspects they have measles, even if the symptoms are mild, should contact their family doctor or call Healthline on 0800 611 116.