Superintendent Des Bray told reporters the dead child, whose sex has not been determined and is believed to have been aged between two and seven, was likely killed elsewhere.
He said the body was stuffed in a suitcase and left alongside the Karoonda Highway, 130 kilometres east of Adelaide, some four to six weeks ago.
By the time it was found by a passing motorist on Tuesday, the remains were about two metres away from the bag.
"It's hard to say what happened, and why, but we know for certain that somebody has come, found the suitcase, opened it to have a look what was in there, tipped out the contents and at that stage become aware that it was most likely human remains."
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Police said it appeared that someone had pulled the suitcase from where it had originally been dumped and placed it in full view of the road.
"It was right in the open and easily able to be seen," Bray said.
Bray said they were looking for a neatly-dressed man, aged about 60, who was seen at Wynarka walking towards the highway some weeks ago carrying a suitcase early one morning.
"We have no description of the clothes he was wearing but he was carrying a dark-coloured suitcase in one hand... He was not a local," he added in comments carried by the ABC.
"There would not be many people walking around Wynarka carrying a suitcase at 7:30 AM in the morning."
"Somebody must know somebody where a child has disappeared in suspicious circumstances," Bray said.
"So we would encourage people to have a think about family, friends, acquaintances, people who live near them, perhaps somebody who has had concerns about children who live nearby and how they might be treated, and all of sudden a child's gone missing.