Schoolchildren in the Northern Rivers area of NSW may have been exposed to measles after a person with the virus recently returned from Asia.

NSW Health is warning people in the state's north to be on alert for symptoms.

The department said schoolchildren on the Singh Company school bus in the Murwillumbah area on February 5, in the morning or afternoon, should monitor for signs of measles.

People who went to the emergency department of Murwillumbah Hospital on February 9 between 1.15pm and 4pm, or on February 10 between 12.15pm and 8pm, may also have been exposed.

The North Coast regional director of population and public health, Valerie Delpech, said in a statement on Sunday night the locations posed no ongoing risk but people who were there at the highlighted times should be alert for symptoms.

They include fever, a runny nose, sore eyes and a cough, usually followed three or four days later by a red, blotchy rash that spreads from the head to the rest of the body.

Dr Delpech said people with symptoms should call ahead to their GP or emergency department.

In January, three people with the virus sparked measles alerts in NSW, Victoria and the ACT.

Unvaccinated babies are at high risk of contracting the virus, while infants, pregnant women and people with weakened immune systems face an increased chance of serious complications, including pneumonia and brain inflammation.