The Repco car park has been cordoned off as detectives process the scene.
Port Lincoln Mayor Brad Flaherty told the ABC the accident was a "terrible, terrible incident that will have an awful impact on the community as a whole and the families involved".
Adelaide MP Nat Cook shared the news on Twitter, branding it an "absolute tragedy", while Senator Justin Hanson wrote: "How could this happen?"
Port Lincoln is on the Eyre Peninsula in South Australia, about a seven-hour drive from the Adelaide CBD, or 251 kilometres directly across the water.