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.

"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.