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Separate outbreaks of norovirus - causing vomiting, diarrhoea, stomach ache and fever - hit hospitals or rest homes in Hamilton and Nelson last week, with ESR expecting another "big year" of reported cases.
Meanwhile, North Shore Hospital is dealing with a "blip" in a separate virus, which attacks the body's ability to respond to antibiotics.
The coronary care extension ward of Waikato Hospital reopened yesterday after 72 hours following a suspected case of norovirus which affected 31 staff and 20 patients.
One patient in the extension ward remains in isolation, but Waikato District Health Board infectious control committee chairman Dr Graham Mills said the hospital and wider community should remain vigilant about hygiene.
"It's just a very hardy little virus that's not fantastically easy to kill."
The source of the virus was human faeces, but it could spread from contact with faeces or vomit. Contact with infected surfaces was a common cause of infection, which was why it was important to wash hands after going to the toilet and before eating food.
At Resthaven retirement home in Cambridge, manager Annie Schenkel said about 20 per cent of the 74 residents had been affected, together with five of the 80 staff members.
The rest home was following DHB guidelines for handling the outbreak, but it was hard to control residents coming and going from their units.
Nursing agencies were limited in how many staff they could provide for cover, as there was considerable risk in sending them on to another facility after time spent at Resthaven.
A Nelson Hospital spokeswoman said the suspected norovirus had struck at the weekend affecting six patients and 11 staff. A surgical ward was closed to new admissions and some surgery had been postponed for 24 hours.
The norovirus has been on the increase for the past two years throughout New Zealand. A surge in cases was also seen in 2004.
Environmental Science and Research communicable disease science leader Gail Greening said she expected this year's statistics to be the same as last year, which had been a "big year" for the virus.
This was a continuation of recent patterns, and was reflected in Australia and Europe. In July and August, Australia had dealt with large outbreaks in Queensland, New South Wales, and Victoria.
In March last year Nelson College was closed after about 100 students were infected with norovirus.
In July last year New Zealand's largest waterborne outbreak of norovirus struck the Cardrona Ski Resort, when water was extracted from a stream downhill from an unlined sewage storage pond, and waste from a septic tank overflowed.
A June ESR surveillance report said 111 of the 245 staff at the skifield and 103 visitors were affected, although the actual numbers were thought to be much higher.
Among the many concerns that arose from the outbreak was that the Medical Officer of Health was not notified (despite the early involvement of a local GP).
Concern was also raised about the slow disclosure of important environmental information, the absence of a staff-sickness policy (with sick staff continuing to work), the shortage of hot water supplies and the fact that the unregistered drinking water supply was managed by an untrained person.
Meanwhile, North Shore Hospital spokeswoman Bryony Hilless said there was no norovirus outbreak on its wards, but there was a "blip" in Extended Spectrum Beta Lactamase (ESBL), a virus resistant to antibiotics.
This was the only symptom of the virus, and it was tested and detected using wound swabs or urine tests.
The hospital had not closed any wards but had put affected patients in isolation or had "cohorted" them with one another. Rigid control measures were in place.
Hand hygiene was important and staff were wearing gloves and gowns to cut risk. Ms Hilless could not specify how many patients were affected.
