Police say they pulled over two speeding vehicles on Brand Highway, near Mingenew, about 7.30pm (WST).
Police say none of the occupants were able to provide proper identification and were detained under the Immigration Act.
Three Malaysian men, a Vietnamese woman and her five-month-old child were taken to Geraldton Police Station.
Sergeant Grant Rosman says police will be handing over the investigation to immigration authorities.
“Immigration Department have now taken over that investigation and looking at their validity to be in Australia,” he said.
Police allege the Vietnamese woman had been illegally working at a plantation in Carnarvon.