Street disorder which erupted during Napier's 32nd annual Art Deco weekend may have been sparked by posts on social media.
First-year Art Deco Trust chairman and Hawke's Bay historian Michael Fowler, said as the huge celebration drew to a close late on Sunday: "The Trust is extremely disappointed in the group of youths' behaviour. We've been informed by reliable sources that this was planned on social media with a view to try and disrupt a very successful family event. "
At least 10 people were arrested after more than 100 drunken youths brawled in Napier on Saturday night.
Some were dressed in retro Art Deco costumes, indicating they had been part of this weekend's festivities.
Police say they had to break up fights involving large gatherings of youths in the Marine Parade area. Violence continued to erupt, even with police present, a spokeswoman said.
A witness described the chaos that erupted.
"There's a lot of police and drunken youths knocking about", a Napier man told the Herald.
"Police seem to be pushing them out of town. They were gravitating towards McDonald's but the kids have all done a runner now."
As further reports came in of more fighting, the man said: "The world's gone mad."
Fowler said a full debrief will take place, including considering what had happened and what action may be taken to prevent a recurrence of the Saturday night arrests which disturbed a festival which boasted about 300 events linked to the architectural and social heritage from the years surrounding the Hawke's Bay earthquake of 1931.
"It is too early to look at specific changes for next year," Fowler said. "But we will undertake a full debrief in the coming weeks. Overall, the Festival has been a huge success."
The trust was calculating foot-traffic at up to 50,000 people across 40 venues over the five days of the festival, of all ages and many dressed in costume of the period bringing yet another stunning display of colour and raimentto the city.
The biggest crowds were those that lined Saturday's parade through the Napier CBD and which flocked to Marine Parade on Friday and Saturday nights.
"We have once again had superb engagement from the community, both young and old, with particular highlights being the Vintage Car Parades, Friday Night with the Stars, the Royal NZ Navy Band, Memorial Flying Displays and the Soap Box Derby," Fowler said.
"We have been delighted with the level of enthusiasm from both the locals and visitors alike," he added. "As a charity, with a huge number of volunteers, we are particularly appreciative of the support received from the wider community".
While there were extremes of weather, from a continuation of the 30C summer temperatures to Saturday night rain and some Sunday wind, there were no cancellations, but events at Hawke's Bay Airport were rearranged after the a light aircraft flipped on Sunday afternoon.