Local health authorities warn the symptoms look like norovirus, and some festival-goers may need to be isolated to stop a full outbreak.
Medical officer of health Dr Bruce Duncan said they needed to stop any further contamination.
“Fifteen young people were transported [this morning] to Gisborne Hospital, where an isolation ward has been set up.
“The priority is avoiding an outbreak. Norovirus has not been confirmed, but it remains a possibility.
“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.”