A spike in serious gastroenteritis cases across NSW prompted a warning from health authorities in July. At its worst point, cases of gastro outstripped influenza during peak flu seasons, with more than 3300 people attending EDs from the start of June to July 20.

Similar outbreaks were reported in WA and the ACT.

NSW Health suspected a new strain of the virus was responsible for the outbreak, which would explain lower immunity levels among the community leading to higher susceptibility. The hardest hit were daycare centres, aged care homes and hospitals.

But the researchers at UNSW were taken aback to discover a trio of virus strains had caused the outbreaks.

Two of the viruses are hybrid strains that evolved from a previous pandemic strain dubbed Sydney 2012, said UNSW PhD student and molecular virologist Jennifer Lun, who worked out the genetic typing of the new viral strains.

The particularly nasty strain was the main source of norovirus cases and major outbreaks until it tapered off from 75 per cent to just 18 per cent of cases in Australia earlier this year.

The third strain most likely came from Asia, Ms Lun said.

"It occurred to me immediately that there was a potential for them to cause an increase in outbreaks this winter, because people have not been exposed to them before," Ms Lun said.

NSW Health has confirmed the new strains are linked to the recent outbreaks.

Professor Peter White, who led the research team at UNSW said now that the prevalence of Sydney 2012 had declined, the three new strains of norovirus had emerged as a new major health concern.

"They are responsible for a big increase in the number of gastro cases in Australia in the past two months, and this new spate of infection is likely to continue to cause a wave of sick leave that will affect businesses and schools already reeling from the effects of the current influenza epidemic,"  said Professor White at the UNSW School of Biotechnology and Molecular Sciences.

Viral gastroenteritis is highly infectious and often spread via direct contact with an infected person and aerosol particles when people vomit.

Symptoms include nausea, vomiting, diarrhoea, fever, abdominal pain, headache and muscle aches, which can take between one and three days to develop and usually last between one and two days, sometimes longer.

Young children and the elderly were most vulnerable, but an abnormal number of young adults presented with gastro last week, suggesting the latest outbreaks were particularly severe.

The new strains have been christened "New Orleans 2009/Sydney 2012", and "GII.P16/SYdney 2012" and "Kawaskai 308".

"During the past 20 years there have been six global epidemics of norovirus, in 1996, 2002, 2004, 2006, 2009 and 2012. Only time will tell how widely these three new strains will spread," Professor White said.

Director of Communicable DIseases at NSW Health Dr Vicky Sheppeard said the prevalence of gastro cases, and the rate of hospital admissions, had returned to the upper range of normal levels for almost all age groups.

But presentations among adults 17 to 34 years of age were still above the levels seen for the past five years.

The latest figures show 3,516 people presented to emergency departments with gastro between Monday 29 August and Sunday 11 September.

A total of 750 of these patients were admitted to hospital, Dr Sheppeard said in a statement.

There were 23 gastro outbreaks reported by child care centres, aged care facilities and hospitals, affecting 243 people over the same two week period.

Specimens have been collected form six outbreak but results have not yet been reported, she said.

NSW Health had previously warned people to stay at home, not visit hospitals or aged-care facilities, and keep children away from school and day care with they were symptomatic.