One large and two smaller slumps had been discovered in the Patea moles rock walls after an inspection in December last year, he said.
The retaining wall was integral to keeping the water from the Patea River tracking behind the moles.
"If there is a breakthrough in the wall the river will change direction and eventually make the river mouth impassable to boaties."
Mr Manning said the maintenance of the Patea moles - or breakwaters - was of prime economic benefit to the Patea and South Taranaki Community.
The moles in the harbour entrance were also of great historical and spiritual value to the Ngati Ruanui, he said.
With the council maintaining the moles, it decreased the risk of gradual reduction of the rock wall or possibly a catastrophic failure if the river flooded or there was major storm surge, he said.
The council planned to bring in an expert consultant in coastal erosion this year to look at the moles.