UPDATED 3.05PM: Immunisation has come into sharp focus as a national outbreak of whooping cough has been declared.

The Ministry of Health is encouraging people to be extra vigilant when they gather for Christmas and New Year celebrations, in order to protect young babies from whooping cough, with 1315 cases reported since the start of the year.

It declared a national outbreak of the highly contagious infection on Friday afternoon.

The best way to prevent the spread was for babies to be immunised, the ministry said.

"Babies under one year old are most vulnerable to the disease and often catch it from older siblings, their parents or family members, and friends," the director of public health, Caroline McElnay, said.

"The best way to protect babies is for pregnant women to get their free immunisation against whooping cough between 28 and 38 weeks of pregnancy, and take their baby for their free immunisations when they're six weeks, three months and five months old.

"If immunisation is delayed, babies are vulnerable for longer."

Outbreaks of the disease occur every three to five years.

New Zealand's most recent national outbreak spanned from August 2011 to December 2013, with about 11,000 cases notified.

Three babies or young children died during the period, with hundreds needing hospital treatment, according to the ministry.

The ministry has also asked midwives and general practices to work together to ensure pregnant women are referred to GPs for immunisation.