Police have downgraded the search for a Riverton fisherman who went missing on Sunday afternoon after the boat he was in sank.

Police have named the man as 28-year-old Jamie Stephen Boniface.

Boniface was in a small dingy with two others when it overturned in the Riverton Estuary around 2.30pm on Sunday.

The police national dive squad carried out an extensive search of the estuary and other areas today after three days of extensive searching.

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"After three days of searching police are now entering a limited continuous search phase," a police spokesperson said.

"Anyone who finds any personal items washed up in the Riverton coastal area is asked to please contact Riverton Police on (03) 234 7070."

On Sunday, a police spokesperson said the two others made it to shore unhurt.

Police Search and Rescue, Riverton Coastguard, LandSAR teams from Invercargill, Te Anau and Tokanui, Riverton Fire Brigade and Southern Lakes Helicopters, along with local vessels and family members were all involved in the search.

Metservice meteorologist Leigh Matheson told the Herald on Sunday evening the weather for the region had been fine and partly cloudy throughout the day.

"Fairly light winds although there's a Westerly that's been picked up in the afternoon.

"About 12 to 13 degrees, pretty quiet, nothing out of the ordinary," she said.