There have now been 25 confirmed cases of measles in New Zealand's South Island as an outbreak continues to spread.

An outbreak of measles in New Zealand's South Island has continued to spread as health authorities order emergency supplies of vaccines.

Authorities on Monday confirmed there had been 25 cases of the disease in the Canterbury region, which includes the country's third-largest city, Christchurch.

"The number of confirmed cases .... is likely to rise further over the coming days and weeks," Canterbury's District Health Board said.

"Measles is now in widespread circulation and people who aren't fully immune could be exposed to the risk of infection anywhere in Canterbury."

Health centres in the region have been in short supply of vaccines since the outbreak in late February, with 3000 doses shipped last week and 18,000 more expected to arrive in coming days.

"Unimmunised people who come within two metres of an infectious person, however briefly, have a 90 per cent chance of contracting measles," the health board said.

All cases of measles in New Zealand since 2012 have originated from overseas arrivals, according to the Ministry of Health.

While the vaccine is considered highly effective, Kiwis in their late teens and early 20s are considered most at risk of contracting the disease because of low update of standard scheduled vaccinations by those in their age bracket.

About 110,000 people died of measles globally in 2017, the World Health Organisation says.