There's growing worries measles is spreading undetected through the country.

Three new cases in Manawatu and Nelson were announced yesterday, bringing the total of current infections to four.

Health New Zealand hasn't been able to confirm how each became infected - and is warning of a potential national outbreak.

Immunisation Advisory Centre medical director, Dr Nikki Turner, says four cases might not sound scary - but measles is highly infectious.

"If we don't know it came from travel - then where did it come from? We do not want this bug spreading through our communities."

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