Cases of norovirus have been confirmed at Auckland Hospital.
Auckland District Health Board (ADHB) said that 33 elderly patients have now been isolated, and infection control measures have been implemented.
ADHB's Margaret Dotchin said 13 of these patients and four staff members showed symptoms of having the virus.
"We are implementing standard infection control measures to prevent the further spread of the virus to patients and staff," she said.
"This means three of our older people's wards are closed to all admissions."
A number of cases of norovirus have been reported recently at Middlemore, Tauranga and Gisborne hospitals.
"ADHB takes infection control very seriously and we are doing everything possible to care for these patients and to minimise the risk of others becoming infected," she said.
Norovirus spreads easily and causes severe stomach upset, resulting in diarrhoea or vomiting.
"Viruses like this do occur in hospital environments from time to time and we have recently seen a number of cases at other hospitals around the Auckland region," said Dotchin.