Ten people are in hospital in Auckland after contracting typhoid and public health officials are investigating whether anyone else has been infected.
The Auckland Regional Public Health Service (ARPHS) has confirmed the outbreak and says it is tracing people who have been in contact with those who have been diagnosed with the disease and "following usual protocols" to stop it spreading further.
Medical Officer of Health Dr David Sinclair said it was unclear how the outbreak started.
"We're trying to work that out. We've got some clues but other things we haven't sorted out yet, so that will take a little while to sort out yet. But in general typhoid is imported. It comes from people returning to New Zealand from the Pacific or from Southeast Asia."
The disease is potentially fatal.
"But that's uncommon now with antibiotic treatment," Sinclair said.
"But that's something we need to be aware of. It can spread quickly and it can be quite a serious illness."
Good hand-washing was the best protection against typhoid, which is mainly spread through water and food but can also spread from person to person.
Shellfish sourced from sea beds contaminated by sewage have caused earlier outbreaks in New Zealand, the Ministry of Health website states.
Flies can also spread the bacteria to food, but most large epidemics are caused by faeces contaminating water supplies.
About 20 to 30 people are diagnosed with typhoid every year in New Zealand, so to have 10 infections in such a short period was a bit concerning, Sinclair said.
"The last significant outbreak in Auckland was in 2013."
He told the Herald the first patient was admitted to hospital at the end of last week.
Doctors realised there had been an outbreak when nine others were confirmed to have the infection and hospitalised this week.
The patients were locals rather than tourists, coming from suburbs as far apart as Blockhouse Bay and South Auckland.
Sinclair was unable to confirm their conditions.
According to the Ministry of Health's website, people with the disease usually present symptoms eight to 14 days after being infected.
Symptoms to look out for were a high fever developing over several days, headaches, muscle aches and "just generally feeling very fatigued and tired".
"Some people also get abdominal pain and constipation. Diarrhoea is uncommon, which is a bit strange for a disease which is spread through food and water.
Sinclair advised anyone feeling very unwell to see their doctor or an after-hours clinic.
ARPHS has notified the medical sector of the outbreak.
The bacteria that causes typhoid is found in the stools of about 10 per cent of untreated patients for more than three months, and 2 to 5 per cent of those infected who aren't treated for typhoid become permanent carriers, according to the Ministry of Health.