The number of people infected with Hepatitis A in Auckland rose by 12 over the weekend, to 31 confirmed cases.
The Regional Public Health Service says the rise in confirmed cases, from 19 on Friday, was expected as more people were tested over the weekend
Medical Offiicer of Health Shanika Perera says the source of the virus is not yet determined, but is likely to be linked to people travelling overseas.
Dr Perera says common symptoms include fever, upset stomach and tiredness.
Nine schools in the region are believed to be at higher risk.