Thieves have taken more than 10 tonnes of valuable scrap metal from a central Balclutha disposal site over the past two and a-half weeks.
Police said the theft, which probably happened over several visits to the riverside site, could be a lucrative move for the thieves, with the rail iron and fishplates worth an estimated $550 a tonne.
The haul could be worth $5500, Constable Graeme Ferguson said.
The metal was taken from a container bin on the banks of the Clutha River behind the Balclutha Railway Station.
The bin, which was not covered, was used as a storage facility for off-cuts of metals by rail company OnTrack.
Const Ferguson said thieves must have taken several trips to have taken all of the metal.
Even small items are very heavy, he said.
Police believe the theft happened some time between August 11 and 25.
