The national mumps outbreak has reached Whanganui with three confirmed cases reported to the Public Health Centre.

More than 900 cases of mumps have now been notified across New Zealand with the majority in Auckland, and most in the 15 to 24 age group.

Whanganui medical officer of health Patrick O'Connor said two adults and one child have been notified with mumps to the Public Health Centre.

Read more: Mumps outbreak: Vaccination catch-up campaign floated for 10-29 year-olds

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Mumps is caused by a virus which can spread when an infected person coughs or sneezes, or passes it on through their saliva when kissing or sharing food and drink.

Antibiotics will not treat the infection or prevent spread.

Dr O'Connor said the usual early signs were fever and headache and then after about two days salivary glands on one or both sides of the jaw may become swollen and sore.

"A person with mumps is considered infectious from two days before the facial swelling until five days after," Dr O'Connor said.

Males after puberty may experience pain and swelling of the testicles, and some females may experience pain in their ovaries. Brain inflammation and one-sided deafness are rare complications.

Dr O'Connor said anyone with these symptoms was asked to stay away from other people for five days after the swelling.

"If another person is infected, it will usually take 16-18 days until the person becomes unwell, but this incubation period can be 12-25 days.

"If someone is unwell with swelling on the side of their face, they should isolate themselves and contact their doctor who can check for mumps by swabbing saliva on the inside of their cheek.

"If you are seeing your doctor for this reason, it helps to phone ahead as this reduces the chance of infecting someone else."

Dr O'Connor said the best protection against mumps is the MMR (measles, mumps, rubella) vaccine.

Children have this vaccine at 15 months and four years.

Anyone in New Zealand born since 1969 is entitled to two free doses of the MMR vaccine.

Those most at risk in the present outbreak are aged 15-24 years. You can check with your doctor if you are unsure whether you have had the two doses.