A 4-year-old boy has been found dead in an effluent pond on a farm on the West Coast of the South Island.

Reuben Nolan was reported missing from a farm in Harihari, Westland, shortly before 1.30pm today.

Police officers, local volunteers, family and friends, launched a search operation shortly after.

In a statement, Police said Reuben was found deceased in an effluent pond at 4pm today.

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"Police extend their condolences to Reuben's family for their tragic loss," the statement read.

Reuben's death would be referred to the Coroner.

His family was being assisted by Victim Support.