Officials said he was driving to the west coast city of Perth, more than 4,100 kilometers (2,500 miles) from Kendall.
The boy may have avoided suspicion from others during his journey because he appears older than his age. Police Detective Inspector Kim Fehon said the boy is about 6 feet (183 centimeters) tall.
On Saturday, he stopped to get fuel at a gas station in the town of Cobar and left without paying, station manager Vamshi Reddy said.
“When I saw him, he looked like a nice person who was maybe 19 or 20,” Reddy said on Monday. “When police told me he was 12 … I was shocked.”
Officials have released no details about why the boy wanted to drive to Perth or whether he would be charged with any crime.