About 200 people at five Northland rest homes have been hit with a highly-infectious stomach bug.

The homes are urging non-essential visitors to stay away to stop it spreading further.

Northland DHB's medical officer of health Jonathan Jarman said an estimated 200 people had been diagnosed with suspected viral gastroenteritis since April 20.

The virus is suspected to be norovirus and lab results due later this week are expected to confirm it, Dr Jarman said.

The outbreak was first notified at Whangarei's Jane Mander Retirement Village, with about 144 residents and staff affected.

Dr Jarman said since then other cases have been notified at Kamo Home and Village, Puriri Court and Parahaki Court in Whangarei and Radius Baycare Home and Hospital at Haruru Falls.

He said the features of the illness are those of norovirus infection - predominantly vomiting and diarrhoea. He said it was a highly infectious disease and easily transmitted from one person to another.

"Outbreaks of suspected norovirus gastroenteritis are not uncommon in rest homes and hospitals due to the close living conditions of residents and patients but it is difficult to control because it is so infectious," Dr Jarman said.

"Norovirus is highly infectious and sometimes being in the same room where someone has vomited is enough to catch the illness. We have been working with the rest homes, whose staff, residents and visitors are required to follow strict infection control measures to reduce the risk to others."

Dr Jarman asked for family members to have patience if they could not visit a relative because of the outbreak.

"Affected rest homes will be putting up signs and will be asking non-essential visitors to stay away," he said.

Norovirus is normally only a mild to moderate illness but the elderly and the very young can have more serious disease.

Symptoms include vomiting, diarrhoea, stomach pains and nausea. People with these symptoms are advised to stay away from other people and see a doctor if the symptoms are severe or the illness does not get better after two days.

"The most effective way you can protect yourself and your family against norovirus is to wash your hands regularly, using plenty of soap, cleaning under the fingernails, rinsing your hands and thoroughly drying them on a clean towel," Dr Jarman said.