About 30 of the grates, used to cover mudtanks in the Dunedin City Council's stormwater system, have been stolen since Wednesday.
Council staff, who suspect a group of organised thieves are at work, have alerted police and the city's scrap metal dealers.
Council roading and maintenance engineer Peter Standring said each of the grates weighed about 30kg and would take two people to lift.
The grates had disappeared in many parts of the city, from Brighton and Mosgiel to Northeast Valley.
The central city was not affected, he said.
It appeared whoever was taking them was targeting quieter streets, rather than busy main roads, he said.
While no accidents or injuries had been reported, the thefts had left holes in city streets that could be a hazard for anyone using the road, Mr Standring warned.
Some of the holes had been found by contractors, while others had been reported by members of the public.
"Our biggest problem is the safety concern - the cyclists, pedestrians, young kids, even cars, falling down and doing serious damage," he said.
So many grates had disappeared in recent days, the council's contractor - Fulton Hogan - had run out of replacements and was having to order more, which were likely to take several days to arrive, he said.
"You keep so many in stock, but you don't expect this many to go missing," he said.
About 17 of the grates had been replaced, but other holes were being covered temporarily by plywood until the new cast-iron grates arrived, he said.
Each new grate cost the council about $250.
The difficulty involved in lifting the heavy grates meant those responsible for the thefts were "serious", he said.
"It would take a couple of people [to lift]. They get quite stuck - they don't just lift off easily.
"It's certainly something organised," he said.
However, it was unlikely any of the city's scrap metal dealers would "go anywhere near" receiving the grates.
"I would expect any reputable companies wouldn't accept a whole pile of council grates as scrap metal," he said.
Anyone witnessing suspicious activity, or with information about the missing grates, was urged to contact Dunedin police, he said.
Scrap metal dealers spoken to yesterday said the grates were unlikely to be purchased by Dunedin companies, but could have been stolen to order.
"As scrap, they are not going to be worth a huge amount - say around $200 per tonne," Sims Pacific Metals branch manager Rob Pape, of Dunedin said.
In theory, similar scrap metal could be melted down for reuse, or on-sold for use as, for example, part of a fireplace or barbecue, or to cover another drain on a private development, he said.
The scrap metal market had recovered recently, although it was "certainly nowhere near the highs of last year," he said.
Senior Sergeant Bruce Ross, of Dunedin, said police were aware of the thefts and were investigating.
