Pinjarra Police have charged two men over the theft of almost 400 metres of copper wiring.
Police discovered the stolen goods on Monday afternoon, with a wholesaler estimating the retail value of the wiring exceeds $30,000.
The 380 metres of ceramic-coated copper wiring is fire-proof for up to two hours.
Read more: Pinjarra copper theft causing concern
Senior Constable Shaun Ukich said they observed something suspicious about a vehicle while on patrol.
"We were doing a patrol out in the industrial area and a car towing a trailer looked like it was heavy," he said.
"That's what we first noticed about it but then it took off at speed so we followed him back to where he eventually stopped and found [the copper] in the trailer.
"We're not sure at this stage where it was stolen from, we're still trying to figure out who it belongs to."
The driver and his passenger, both men in their 40s, have both been charged.
The bust comes after the Mandurah Mail reported on a spate of copper thefts in Pinjarra in January.
At the time, Senior Sergeant Ian Francis said copper cabling, wire and pipes was being stolen from industrial sites for its expensive street value.
"It is worth hundreds and thousands of dollars," he said.
"There's a market for recycled copper so people are stealing it from construction and industrial sites around the area."
Anyone with information is urged to call the Pinjarra Police Station on 9531 7111 between 8am and 4pm on weekdays or Crime Stoppers on 1800 333 000. You can also make an anonymous report on their website.