The Auckland Regional Public Health Service is investigating a cluster of almost 20 hepatitis A cases in the region.
It says 19 people are confirmed as having the illness, including school children.
The service says it is working closely with the schools involved.
A medical officer of health, Shanika Perera, says hepatitis A is generally a mild illness in children, with no ongoing health effects.
Dr Perera says symptoms in children usually include fever, an upset stomach, and feeling tired and generally unwell.
She says if a parent is concerned their child may have hepatitis A they should keep them away from school, childcare centres and social events and contact their doctor.
Dr Perera says authorities do not know how the disease was contracted initially, but are working with patients to determine its source and prevent it spreading further.