All 17 have been tested.
The remaining contact is being actively followed up by contact tracers and will be advised to get a test.
Of the 18 contacts identified, nine are in Northland, seven are in Auckland, one is in Wellington, and one is still to be determined.
Northland DHB has stepped up testing and screening at Northland hospitals for visitors and patients.
Anyone with symptoms should get tested.
Director of Public Health Dr Caroline McElnay says this would help limit any potential spread of infection from the case.
Public health interviews with the case are underway to confirm their movements, Dr McElnay said.
The DHBs are also encouraging vaccination at general practice, pharmacy, Māori health providers and DHB clinics.