Health authorities in Auckland have recorded five cases of rubella, including three in the past week.
Rubella is a rare virus which is dangerous to pregnant women and their foetuses.
It is otherwise known as German measles and can be protected against by taking the MMR vaccine.
Rubella is rarely seen in New Zealand, with just confirmed 30 cases in the past decade.
The Auckland District Health Board says its been unable to find a link among the five men who have contracted the illness this winter.
The DHB is urging pregnant women to make sure they are immunised to protect themselves and their foetuses.
Medical officer of health Richard Hoskins says the reappearance of the virus is being taken very seriously.
"Rubella is a real danger for susceptible pregnant women that can lead to really tragic outcomes and lifelong disability for the baby."
Meanwhile, Dr Hoskins says the board is also continuing to monitor an outbreak of measles which has reached 110 cases.
The DHB says all but one of the cases appear to have originated from one person who flew to New Zealand from Britain in May.