Thieves have been taking steel from floodgates on the lower Piako River on the Hauraki Plains.
Two walkway platforms have been removed from floodgates in Ngātea, and an attempt was made to steal a third, the Waikato Regional Council said.
One of the platforms was seven metres long and the other 4.5m long.
They were heavy, up high and would have needed at least two people to lift them, the council said.
The platforms were bolted to beams and allowed staff to safely access the floodgates for maintenance work.
"I'd say someone is using them for a walkway across a drain or for scrap metal," Ian Sarah from the council said.
It was noticed on Wednesday that they were missing, during a monthly check of flood protection systems.
The council said the removal of the platforms did not compromise flood protection, but it is a health and safety issue for staff.
The replacement platforms will be welded on to prevent further thefts.
"We have in excess of 120 steel walkways on the Piako-Waihou scheme, and any adjacent to highways and roads will be welded down," Mr Sara said.
The police had been informed of the thefts.
"If you've seen a nice walkway bridge somewhere over a drain, or any suspicious activity, it'd be good to let them know."
The council was treating it as a serious incident, not only because of the cost on ratepayers, but also because it threatened lives and the integrity of critical infrastructure that was there to prevent damage to properties during floods.