Tuesday's heavy rain mainly affected farms on river flats near the mouths of the Whangaehu and Turakina rivers, Federated Farmers' Wanganui president Brian Doughty says.
Surface flooding of the low lying flats may have wrecked some crops.
Inland, the big dump made the land slip, destroying tracks, roads, culverts and fences. Swollen streams affected bridges, and surface flooding will make docking more difficult.
"We are sympathetic to those who have suffered loss and damage from one of the biggest rainfall events I can recall," Mr Doughty said.
His Brunswick farm received about 80mm of rain. The largest amount he's heard of was the 120mm that fell at Orautoha, near Raetihi.