Mr Hamilton said he and a fellow motorcyclist had ridden to the festival to check out a friend's motorcycle concept which was on display at the event.
The pair parked down a service lane behind the back of the shops near the schoolyard where the festival hosted pony rides about midday.
"There were lots of people in the schoolyard because of the pony rides and I thought the bikes were very visual."
Mr Hamilton said the two men went to check out the festival for about 40 minutes and when they returned his bike was gone.
"A young fella who was working on the horses came up to me and said he was sure some guys had pulled it into a van and looked suspicious."
He said a mother had also spotted suspicious behaviour and taken photographs which she had handed into police.
The van was green with distinctive markings on the side, Mr Hamilton said.
"It had big red graffiti writing on the side with 'for sale' on it."
Mr Hamilton said he had bought the silver and black bike about five or six years ago and it was worth about $15,000.
"It is a classic," he said. "It is instantly recognisable. It is the only one that is painted in those colours."
He said he had put a lot of time and effort into it. "It is not a blingy bike," he said. "I ride it regularly. It has been ridden all over the North and South Island. It is a good, reliable bike."
Greerton Mainstreet manager Sally Benning said she had not heard about a bike being stolen from the festival and was disappointed to hear what had happened.
A police media spokeswoman said police were alerted to the incident at 12.50pm Saturday.
"Members of the public reported seeing a dark coloured van pull up beside the motorbike and two people put the bike inside the van and drive off," the spokeswoman said.
Police said enquiries were ongoing.
STOLEN:
What: Black and silver 1973 Norton Commando
When: September 30
Number plate: 36TXP
Anyone who was in the area at the time, or may have information that could help is being urged to call Tauranga Police on 07 577 4300 or Crimestoppers anonymously on 0800 555 111.