Police say a 73-year-old cyclist killed on a highway south-east of Adelaide was run over by at least six trucks.

The man was cycling on the Dukes Highway, five kilometres west of Coonalpyn on Monday night.

Inspector Philip Newitt from the major crash investigation branch says three truck drivers have provided information to police, but other truck drivers could be interstate and unaware of the investigation.

He says police want to speak to truck drivers who had radio discussions about what happened.

"Some of the witnesses we have spoken to thus far have stated to police that discussion on the high frequency radio centred on a man riding a bike near Coonalpyn," Inspector Newitt said.

"It is obviously great that truck drivers are warning each other of potential dangers and we certainly encourage that. We would like to take that a step further and for those truck drivers to come forward."