A regional Australian oyster festival is challenging the Guinness world records’ definition of “biggest oyster” after its prize contestant – a 2kg mollusc named Jack – was deemed too short.

The organisers of the Narooma oyster festival say Guinness uses a misguided method that unfairly judges competitors on length rather than weight.

“The reason they don’t judge on weight is because it could be harmful to animals – you can imagine cats and dogs and people’s pets can be overfed,” said Cath Peachey, a spokeswoman for the festival. “But you can’t actually overfeed an oyster. The length of the shell alone is not a true indication of size.”

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Jack’s grower, Bernie Connell, said the rules should be rewritten so his “perfect” oyster could claim the crown.

“He’s the heaviest oyster and the widest oyster and the deepest oyster, so three out of four is pretty good,” Connell said. “All things fair they ought to give us the record.

“Jack is the perfect example of an oyster: excellent in his looks and well proportioned. You need to know a lot about oysters to get it, but he’s lovely and round, with lovely curves. He’s the perfect shape, like a boat.”

At an official event in Narooma in May, Jack weighed in at 1.9kg, 300g heavier than the current champion, a Danish oyster from the Wadden sea. However, Jack was only 27cm long, compared with the record 35.5cm.

The festival has written to Guinness to formally request that the width, depth and weight of an oyster should be considered, but does not expect a reply for three months.

Guinness requires any measurement be taken three times and averaged, in front of two witnesses, with a veterinary statement that the oyster is healthy and over a year old. The process must be filmed and the oyster “moved on the ground between each measurement”.

The festival complied with all requirements.

Meanwhile Connell is confident Jack will continue to grow. He is only four, and the Danish champion was estimated to be at least 15 when it broke the record in 2013. The Danish oyster is also a cluster; Jack is a single oyster.

His plan is to return him to the Clyde river and “not go near him”.

“He’s a lovely single oyster,” he said. “He doesn’t have the assistance of other oysters growing off him to make his weight.

“I don’t think he’ll grow much longer but he’ll grow a lot heavier. He could get another inch thicker in the next three or four years. It should be enough to win it.”