Tiki was one of two boats swept from their moorings in Karitane on Tuesday during the downpour that caused disruption throughout the South.
Both boats ended up on nearby Waikouaiti Beach.
Retired fisherman Roger Bartlett said Tiki had "split open like a hard-boiled egg" and was beyond salvaging.
"It’s just a matter of stopping any stuff drifting away, [we’ve] secured the site and locked her up and I’m about to get hold of the insurance company."
Tiki was 60 years old and he had owned it for nearly 50 years.
"The boats that weren’t secured to the bank all got carried away, except for mine — she was the last one left on the river, on the mooring."
He had used the same mooring system both times, and put his loss down to bad luck.
"A lot of garbage came down the river this time, like trees and hay bales, and I think something must have clubbed the mooring rope and broken it.
"It was a bit of an ignominious way for the old girl to go, but there it is."
His main vessel for three decades as a fisherman, he had continued to use Tiki recreationally, but had no plans for a replacement.
"I’ll still continue with trout and stuff."
Catamaran First Light was also swept away on Tuesday and despite a mammoth effort to dig the yacht free yesterday, it remained stuck fast.
Owner Alvin Frew estimated up to five tonnes of sand could be weighing down the hull.
"We may have to cut the boat up ... Tomorrow’s another attempt," he said.
He said his three mooring ropes had "snapped like a bit of cotton".
