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The dahlia, about 25cm across, was entered by Waikouaiti couple Wayne and Donna Burgess.
Mr Morton described it as "beautiful" and said several other flowers were also unusually large in size.
"We haven't had the bigger [dahlias] like that before in quite a few years," Mr Morton said.
Rows of roses, cacti and succulents and elaborate, delicate flower arrangements were among more than 400 entries on display at the show, which was held at the Forbury Park Raceway.
"There were over 200 entries in the dahlias [and] there were probably 220 entries in the horticultural show," he said.