South Australia's consumer watchdog has issued a public warning against a Royal Park-based custom trailer builder, following numerous complaints from consumers.

Commissioner for Consumer Affairs Dini Soulio said Consumer and Business Services had received four complaints from consumers who had spent a total of more than $40,000 on trailers sold by Daniel Pittaway, trading as Dannyway Customs.

He said the builder had also operated from premises at Burton and Christies Beach.

"In each case, the consumers say they were provided with vehicles that were not roadworthy," he said.

"In three matters, the trailers were inspected by staff from the Department for Infrastructure and Transport - and all three custom-built trailers were found to have significant faults that made them un-roadworthy and they were not economical to fix."

Mr Soulio said consumers should avoid dealing with the business.

"These are not complaints about minor defects - they're complaints about significant flaws involving trailers that should not be on the road, and pose a significant safety risk to both the drivers and members of the general public," he said.

"CBS is continuing to investigate these complaints, and will assess whether further action is warranted."

The public warning can be found here.

To report a concern about a trader, contact Consumer and Business Services on 131 882.