A third case of measles has been confirmed in Nelson.
It's the first to require hospitalisation but is not related to two earlier cases, Nelson Marlborough Public Health Service says.
"The best thing for people to do is to make sure you and your children are up to date with the MMR [measles mumps rubella] vaccine," medical officer of health Dr Andrew Lindsay said.
"Check with your GP to be sure, and your GP can also give you the best immunisation advice for your circumstances."
He said measles is highly infectious to non-immune people but people born before 1969, and people who have had two doses of the MMR vaccine, are "almost certainly" immune to measles.
People who have definitely had measles in the past are also immune.
The first two cases were related, as the second case was one of the four people in isolation who had been in contact with the first case.
The first case is now recovering and is no longer contagious.
Regional GPs are on alert to possible cases presenting and will assess and notify the Public Health Service if appropriate.
Nelson Marlborough Health is working with the Ministry of Health and other DHBs to develop a co-ordinated response to the outbreak.
Measles starts with a fever and usually a cough or runny nose, and perhaps sore, red eyes before the rash appears.