Four Northland rest homes remain under public health surveillance after an outbreak of a highly infectious virus.

So far there have been 283 confirmed or suspected cases of norovirus gastroenteritis, with two linked deaths in the district.

Northland District Health Board's medical officer of health Jonathan Jarman said norovirus was a highly infectious disease and was particularly difficult to control in rest homes.

"Outbreaks of suspected norovirus gastroenteritis are not uncommon in rest homes and hospitals due to the close living conditions of residents and patients.

"It's highly infectious and sometimes being in the same room where someone has vomited is enough to catch the illness."

However, the infection control measures were working as one rest home -- Jane Mander in Whangarei -- was now outbreak free, he said.

The four other homes -- Kamo Home and Village, Parahaki Court, Puriri Court, and Shalom Aged Care, all in Whangarei -- were managing to keep their case numbers low, Dr Jarman said.

"We have been looking for possible causes but it appears that what we are seeing in rest homes is just a reflection of what is happening in the community," said Dr Jarman.

The two deaths were linked to the norovirus outbreak, but it was not the cause of them.

"Norovirus gastroenteritis is normally only a mild to moderate illness but like most diseases it tends to hit the elderly harder."

Symptoms include vomiting, diarrhoea and stomach pains.