Fifteen people are in Gisborne Hospital with severe vomiting and diarrhoea, after attending Rhythm & Vines.

Fifteen Rhythm and Vines festival-goers are in Gisborne Hospital with severe vomiting and diarrhoea, which health authorities fear could be from norovirus.

St John Ambulance paramedics treated people in a large tent set up on the site, but the number of people presenting with norovirus-like symptoms meant they needed to be isolated to avoid contamination, Tairāwhiti District Health Board medical officer of health Bruce Duncan said.

* Were you affected by the bug? Contact newstips@stuff.co.nz

TAYLA OMEARA/STUFF The Rhythm & Vines festival site near Gisborne was a sea of mud on day one, on Friday.

"The priority is avoiding an outbreak. Norovirus has not been confirmed, but it remains a possibility.

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"With thousands of people in close proximity, it was a priority to do all we can to prevent a mass outbreak. At this stage, this appears to have been successful."

TAYLA OMEARA/STUFF The site at Waiohika Estate on day one.

The three-day music event, which began on December 29 and ended on Monday morning, attracts thousands of revellers from around the country every year.

This year's 15th instalment was no different, but torrential rain turned tracks to the festival's stages into mud.

St John paramedic Shane Clapperton was one of 30 providing medical care on New Year's Eve, and said the outbreak began in the early hours of January 1.

The worst cases were then taken to Gisborne Hospital, he said.

"They weren't serious, they were moderate, but required transport to hospital so they could be managed before they leave town."

An isolation ward was set up in the hospital for those affected by the stomach bug.

Paramedics treated just over 400 people throughout the celebrations for issues ranging from intoxication, to cuts and sprains, and dehydration.

Anyone who has left the site, but suspects they might have the bug, is urged to contact Healthline 0800 0800 611 116 in the first instance for advice. A qualified nurse is at the end of the phone around the clock.

"If anyone is sick, it is important to drink plenty of water," the DHB said. "Please make sure you wash hands with soap and water and thoroughly dry them before handling or eating food, and after going to the toilet."

Norovirus symptoms usually last between one and three days.