Six staff members and seven patients at Auckland City Hospital have tested positive for Covid-19.
A staff member from one of the hospital's older people's wards recently became symptomatic and returned a positive result.
Staff and patients in the ward were tested, and another 12 cases were found.
"We immediately activated our plans to prevent the spread of the virus within the hospital and worked quickly to test staff and patients on the ward," Auckland DHB's director of adult community and long-term conditions Samantha Titchener said in a statement.
"As at 6pm Saturday 12 February, we've identified that five staff members and seven patients from the Awatea Ward and one staff member from the Marino Ward have tested positive for Covid-19. The source of transmission and index case is under investigation."
"We're grateful that our staff member took prompt action and was tested when they became symptomatic.
"We've contacted the families of patients who have returned a positive Covid-19 test and are supporting them and their loved-ones.
"We've been preparing for the likely impact of Omicron on our hospitals and teams, including high numbers of people with Covid-19 in our hospitals, and for some of our staff to be off work if they become unwell or need to isolate. This is not unexpected as Covid-19 cases in the community increase.
"We would like to assure the public that patient safety is our utmost priority and we're doing everything we can to keep our patients and staff safe. We have full confidence in our measures and our staff to manage this situation safely.
"We have robust infection prevention and control measures in place and we work closely with occupational health and follow their guidelines, for everyone's safety."
She said Auckland DHB patients and whānau who had been at Auckland City Hospital did not need to take action unless contacted by public health or the DHB.